diff --git a/app/Http/Middleware/HandleAppearance.php b/app/Http/Middleware/HandleAppearance.php
index 1a01684..de0b96b 100644
--- a/app/Http/Middleware/HandleAppearance.php
+++ b/app/Http/Middleware/HandleAppearance.php
@@ -16,7 +16,11 @@ class HandleAppearance
      */
     public function handle(Request $request, Closure $next): Response
     {
-        View::share('appearance', $request->cookie('appearance') ?? 'system');
+        $appearance = $request->cookie('appearance');
+        $appearance = in_array($appearance, ['light', 'dark', 'system'], true) ? $appearance : 'system';
+
+        $request->attributes->set('appearance', $appearance);
+        View::share('appearance', $appearance);
 
         return $next($request);
     }
diff --git a/app/Http/Middleware/HandleInertiaRequests.php b/app/Http/Middleware/HandleInertiaRequests.php
index d43d301..d980988 100644
--- a/app/Http/Middleware/HandleInertiaRequests.php
+++ b/app/Http/Middleware/HandleInertiaRequests.php
@@ -46,6 +46,11 @@ class HandleInertiaRequests extends Middleware
         return [
             ...parent::share($request),
             'name' => config('app.name'),
+            'appearance' => $request->attributes->get('appearance', 'system'),
+            'localization' => [
+                'locale' => app()->getLocale(),
+                'timeZone' => config('app.timezone'),
+            ],
             'auth' => [
                 'permissions' => [
                     'canManageTeams' => $user !== null && ($this->permissionService->canManageAnyTeam($user) || $this->permissionService->allows($user, Permission::TeamCreate)),
diff --git a/public/fonts/inter/InterVariable-Italic.woff2 b/public/fonts/inter/InterVariable-Italic.woff2
new file mode 100644
index 0000000..b3530f3
Binary files /dev/null and b/public/fonts/inter/InterVariable-Italic.woff2 differ
diff --git a/public/fonts/inter/InterVariable.woff2 b/public/fonts/inter/InterVariable.woff2
new file mode 100644
index 0000000..5a8d3e7
Binary files /dev/null and b/public/fonts/inter/InterVariable.woff2 differ
diff --git a/public/fonts/inter/LICENSE.txt b/public/fonts/inter/LICENSE.txt
new file mode 100644
index 0000000..9b2ca37
--- /dev/null
+++ b/public/fonts/inter/LICENSE.txt
@@ -0,0 +1,92 @@
+Copyright (c) 2016 The Inter Project Authors (https://github.com/rsms/inter)
+
+This Font Software is licensed under the SIL Open Font License, Version 1.1.
+This license is copied below, and is also available with a FAQ at:
+http://scripts.sil.org/OFL
+
+-----------------------------------------------------------
+SIL OPEN FONT LICENSE Version 1.1 - 26 February 2007
+-----------------------------------------------------------
+
+PREAMBLE
+The goals of the Open Font License (OFL) are to stimulate worldwide
+development of collaborative font projects, to support the font creation
+efforts of academic and linguistic communities, and to provide a free and
+open framework in which fonts may be shared and improved in partnership
+with others.
+
+The OFL allows the licensed fonts to be used, studied, modified and
+redistributed freely as long as they are not sold by themselves. The
+fonts, including any derivative works, can be bundled, embedded,
+redistributed and/or sold with any software provided that any reserved
+names are not used by derivative works. The fonts and derivatives,
+however, cannot be released under any other type of license. The
+requirement for fonts to remain under this license does not apply
+to any document created using the fonts or their derivatives.
+
+DEFINITIONS
+"Font Software" refers to the set of files released by the Copyright
+Holder(s) under this license and clearly marked as such. This may
+include source files, build scripts and documentation.
+
+"Reserved Font Name" refers to any names specified as such after the
+copyright statement(s).
+
+"Original Version" refers to the collection of Font Software components as
+distributed by the Copyright Holder(s).
+
+"Modified Version" refers to any derivative made by adding to, deleting,
+or substituting -- in part or in whole -- any of the components of the
+Original Version, by changing formats or by porting the Font Software to a
+new environment.
+
+"Author" refers to any designer, engineer, programmer, technical
+writer or other person who contributed to the Font Software.
+
+PERMISSION AND CONDITIONS
+Permission is hereby granted, free of charge, to any person obtaining
+a copy of the Font Software, to use, study, copy, merge, embed, modify,
+redistribute, and sell modified and unmodified copies of the Font
+Software, subject to the following conditions:
+
+1) Neither the Font Software nor any of its individual components,
+in Original or Modified Versions, may be sold by itself.
+
+2) Original or Modified Versions of the Font Software may be bundled,
+redistributed and/or sold with any software, provided that each copy
+contains the above copyright notice and this license. These can be
+included either as stand-alone text files, human-readable headers or
+in the appropriate machine-readable metadata fields within text or
+binary files as long as those fields can be easily viewed by the user.
+
+3) No Modified Version of the Font Software may use the Reserved Font
+Name(s) unless explicit written permission is granted by the corresponding
+Copyright Holder. This restriction only applies to the primary font name as
+presented to the users.
+
+4) The name(s) of the Copyright Holder(s) or the Author(s) of the Font
+Software shall not be used to promote, endorse or advertise any
+Modified Version, except to acknowledge the contribution(s) of the
+Copyright Holder(s) and the Author(s) or with their explicit written
+permission.
+
+5) The Font Software, modified or unmodified, in part or in whole,
+must be distributed entirely under this license, and must not be
+distributed under any other license. The requirement for fonts to
+remain under this license does not apply to any document created
+using the Font Software.
+
+TERMINATION
+This license becomes null and void if any of the above conditions are
+not met.
+
+DISCLAIMER
+THE FONT SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT
+OF COPYRIGHT, PATENT, TRADEMARK, OR OTHER RIGHT. IN NO EVENT SHALL THE
+COPYRIGHT HOLDER BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+INCLUDING ANY GENERAL, SPECIAL, INDIRECT, INCIDENTAL, OR CONSEQUENTIAL
+DAMAGES, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+FROM, OUT OF THE USE OR INABILITY TO USE THE FONT SOFTWARE OR FROM
+OTHER DEALINGS IN THE FONT SOFTWARE.
diff --git a/resources/css/app.css b/resources/css/app.css
index 6f9d1f5..2949d41 100644
--- a/resources/css/app.css
+++ b/resources/css/app.css
@@ -1,56 +1,98 @@
 @import 'tailwindcss';
-
 @import 'tw-animate-css';
+@import './theme.css';
 
 @source '../../vendor/laravel/framework/src/Illuminate/Pagination/resources/views/*.blade.php';
 @source '../../storage/framework/views/*.php';
+@source not '../../bootstrap/ssr';
 
-@custom-variant dark (&:is(.dark *));
+@custom-variant dark (&:where(.dark, .dark *));
+
+@font-face {
+    font-family: Inter;
+    font-style: normal;
+    font-weight: 100 900;
+    font-display: swap;
+    src: url('/fonts/inter/InterVariable.woff2') format('woff2');
+}
+
+@font-face {
+    font-family: Inter;
+    font-style: italic;
+    font-weight: 100 900;
+    font-display: swap;
+    src: url('/fonts/inter/InterVariable-Italic.woff2') format('woff2');
+}
 
 @theme inline {
-    --font-sans:
-        Instrument Sans, ui-sans-serif, system-ui, sans-serif,
-        'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol',
-        'Noto Color Emoji';
+    --font-sans: Inter, ui-sans-serif, system-ui, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji';
 
-    --radius-lg: var(--radius);
-    --radius-md: calc(var(--radius) - 2px);
-    --radius-sm: calc(var(--radius) - 4px);
+    --radius-sm: 0.125rem;
+    --radius-md: var(--radius);
+    --radius-lg: 0.5rem;
+    --radius-xl: 0.75rem;
+
+    --spacing-page: 1.5rem;
+    --spacing-column: 1rem;
+    --spacing-card: 0.75rem;
+    --spacing-stack: 0.5rem;
+    --spacing-control: 2.75rem;
+
+    --text-headline: 1.5rem;
+    --text-headline--line-height: 2rem;
+    --text-headline--font-weight: 600;
+    --text-headline--letter-spacing: -0.02em;
+    --text-headline-mobile: 1.25rem;
+    --text-headline-mobile--line-height: 1.75rem;
+    --text-headline-mobile--font-weight: 600;
+    --text-headline-mobile--letter-spacing: -0.02em;
+    --text-title: 1.125rem;
+    --text-title--line-height: 1.5rem;
+    --text-title--font-weight: 600;
+    --text-title--letter-spacing: -0.01em;
+    --text-body: 0.875rem;
+    --text-body--line-height: 1.25rem;
+    --text-caption: 0.8125rem;
+    --text-caption--line-height: 1.125rem;
+    --text-label: 0.6875rem;
+    --text-label--line-height: 1rem;
+    --text-label--font-weight: 700;
+    --text-label--letter-spacing: 0.05em;
+
+    --shadow-card-hover: 0 4px 12px rgb(0 0 0 / 0.08);
 
     --color-background: var(--background);
     --color-foreground: var(--foreground);
-
     --color-card: var(--card);
     --color-card-foreground: var(--card-foreground);
-
     --color-popover: var(--popover);
     --color-popover-foreground: var(--popover-foreground);
-
     --color-primary: var(--primary);
     --color-primary-foreground: var(--primary-foreground);
-
     --color-secondary: var(--secondary);
     --color-secondary-foreground: var(--secondary-foreground);
-
     --color-muted: var(--muted);
     --color-muted-foreground: var(--muted-foreground);
-
     --color-accent: var(--accent);
     --color-accent-foreground: var(--accent-foreground);
-
     --color-destructive: var(--destructive);
     --color-destructive-foreground: var(--destructive-foreground);
-
     --color-border: var(--border);
     --color-input: var(--input);
     --color-ring: var(--ring);
-
+    --color-brand: var(--brand);
+    --color-brand-foreground: var(--brand-foreground);
+    --color-success: var(--success);
+    --color-success-foreground: var(--success-foreground);
+    --color-warning: var(--warning);
+    --color-warning-foreground: var(--warning-foreground);
+    --color-info: var(--info);
+    --color-info-foreground: var(--info-foreground);
     --color-chart-1: var(--chart-1);
     --color-chart-2: var(--chart-2);
     --color-chart-3: var(--chart-3);
     --color-chart-4: var(--chart-4);
     --color-chart-5: var(--chart-5);
-
     --color-sidebar: var(--sidebar-background);
     --color-sidebar-foreground: var(--sidebar-foreground);
     --color-sidebar-primary: var(--sidebar-primary);
@@ -61,112 +103,35 @@
     --color-sidebar-ring: var(--sidebar-ring);
 }
 
-/*
-  The default border color has changed to `currentColor` in Tailwind CSS v4,
-  so we've added these compatibility styles to make sure everything still
-  looks the same as it did with Tailwind CSS v3.
-
-  If we ever want to remove these styles, we need to add an explicit border
-  color utility to any element that depends on these defaults.
-*/
-@layer base {
-    *,
-    ::after,
-    ::before,
-    ::backdrop,
-    ::file-selector-button {
-        border-color: var(--color-gray-200, currentColor);
-    }
-}
-
-@layer utilities {
-    body,
-    html {
-        --font-sans:
-            'Instrument Sans', ui-sans-serif, system-ui, sans-serif,
-            'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol',
-            'Noto Color Emoji';
-    }
-}
-
-:root {
-    --background: hsl(0 0% 100%);
-    --foreground: hsl(0 0% 3.9%);
-    --card: hsl(0 0% 100%);
-    --card-foreground: hsl(0 0% 3.9%);
-    --popover: hsl(0 0% 100%);
-    --popover-foreground: hsl(0 0% 3.9%);
-    --primary: hsl(0 0% 9%);
-    --primary-foreground: hsl(0 0% 98%);
-    --secondary: hsl(0 0% 92.1%);
-    --secondary-foreground: hsl(0 0% 9%);
-    --muted: hsl(0 0% 96.1%);
-    --muted-foreground: hsl(0 0% 45.1%);
-    --accent: hsl(0 0% 96.1%);
-    --accent-foreground: hsl(0 0% 9%);
-    --destructive: hsl(0 84.2% 60.2%);
-    --destructive-foreground: hsl(0 0% 98%);
-    --border: hsl(0 0% 92.8%);
-    --input: hsl(0 0% 89.8%);
-    --ring: hsl(0 0% 3.9%);
-    --chart-1: hsl(12 76% 61%);
-    --chart-2: hsl(173 58% 39%);
-    --chart-3: hsl(197 37% 24%);
-    --chart-4: hsl(43 74% 66%);
-    --chart-5: hsl(27 87% 67%);
-    --radius: 0.5rem;
-    --sidebar-background: hsl(0 0% 98%);
-    --sidebar-foreground: hsl(240 5.3% 26.1%);
-    --sidebar-primary: hsl(0 0% 10%);
-    --sidebar-primary-foreground: hsl(0 0% 98%);
-    --sidebar-accent: hsl(0 0% 94%);
-    --sidebar-accent-foreground: hsl(0 0% 30%);
-    --sidebar-border: hsl(0 0% 91%);
-    --sidebar-ring: hsl(217.2 91.2% 59.8%);
-    --sidebar: hsl(0 0% 98%);
-}
-
-.dark {
-    --background: hsl(0 0% 3.9%);
-    --foreground: hsl(0 0% 98%);
-    --card: hsl(0 0% 3.9%);
-    --card-foreground: hsl(0 0% 98%);
-    --popover: hsl(0 0% 3.9%);
-    --popover-foreground: hsl(0 0% 98%);
-    --primary: hsl(0 0% 98%);
-    --primary-foreground: hsl(0 0% 9%);
-    --secondary: hsl(0 0% 14.9%);
-    --secondary-foreground: hsl(0 0% 98%);
-    --muted: hsl(0 0% 16.08%);
-    --muted-foreground: hsl(0 0% 63.9%);
-    --accent: hsl(0 0% 14.9%);
-    --accent-foreground: hsl(0 0% 98%);
-    --destructive: hsl(0 84% 60%);
-    --destructive-foreground: hsl(0 0% 98%);
-    --border: hsl(0 0% 14.9%);
-    --input: hsl(0 0% 14.9%);
-    --ring: hsl(0 0% 83.1%);
-    --chart-1: hsl(220 70% 50%);
-    --chart-2: hsl(160 60% 45%);
-    --chart-3: hsl(30 80% 55%);
-    --chart-4: hsl(280 65% 60%);
-    --chart-5: hsl(340 75% 55%);
-    --sidebar-background: hsl(0 0% 7%);
-    --sidebar-foreground: hsl(0 0% 95.9%);
-    --sidebar-primary: hsl(360, 100%, 100%);
-    --sidebar-primary-foreground: hsl(0 0% 100%);
-    --sidebar-accent: hsl(0 0% 15.9%);
-    --sidebar-accent-foreground: hsl(240 4.8% 95.9%);
-    --sidebar-border: hsl(0 0% 15.9%);
-    --sidebar-ring: hsl(217.2 91.2% 59.8%);
-    --sidebar: hsl(240 5.9% 10%);
-}
-
 @layer base {
     * {
-        @apply border-border outline-ring/50;
+        @apply border-border;
     }
+
     body {
-        @apply bg-background text-foreground;
+        @apply bg-background font-sans text-body text-foreground antialiased;
+        font-optical-sizing: auto;
+    }
+
+    :focus-visible {
+        outline: 2px solid var(--ring);
+        outline-offset: 3px;
+    }
+
+    ::selection {
+        background: var(--brand);
+        color: var(--brand-foreground);
+    }
+}
+
+/* Preserve completion events while removing visible nonessential motion. */
+@media (prefers-reduced-motion: reduce) {
+    *,
+    *::before,
+    *::after {
+        animation-duration: 0.01ms !important;
+        animation-iteration-count: 1 !important;
+        transition-duration: 0.01ms !important;
+        scroll-behavior: auto !important;
     }
 }
diff --git a/resources/css/theme.css b/resources/css/theme.css
new file mode 100644
index 0000000..b7e8449
--- /dev/null
+++ b/resources/css/theme.css
@@ -0,0 +1,96 @@
+/* Fluid Emerald / Kinetic Kanban: shared by the first paint and the app bundle. */
+:root {
+    color-scheme: light;
+    --radius: 0.25rem;
+    --background: #f7f9fb;
+    --foreground: #191c1e;
+    --card: #ffffff;
+    --card-foreground: #191c1e;
+    --popover: #ffffff;
+    --popover-foreground: #191c1e;
+    --primary: #006e2f;
+    --primary-foreground: #ffffff;
+    --secondary: #d0e1fb;
+    --secondary-foreground: #38485d;
+    --muted: #f2f4f6;
+    --muted-foreground: #505f76;
+    --accent: #dcfce7;
+    --accent-foreground: #005321;
+    --destructive: #ba1a1a;
+    --destructive-foreground: #ffffff;
+    --border: #e2e8f0;
+    --input: #6d7b6c;
+    --ring: #006e2f;
+    --brand: #22c55e;
+    --brand-foreground: #002109;
+    --success: #dcfce7;
+    --success-foreground: #005321;
+    --warning: #fef3c7;
+    --warning-foreground: #78350f;
+    --info: #dbeafe;
+    --info-foreground: #1e40af;
+    --chart-1: #006e2f;
+    --chart-2: #505f76;
+    --chart-3: #9e4036;
+    --chart-4: #92400e;
+    --chart-5: #1d4ed8;
+    --sidebar-background: #f2f4f6;
+    --sidebar-foreground: #191c1e;
+    --sidebar-primary: #006e2f;
+    --sidebar-primary-foreground: #ffffff;
+    --sidebar-accent: #dcfce7;
+    --sidebar-accent-foreground: #005321;
+    --sidebar-border: #e2e8f0;
+    --sidebar-ring: #006e2f;
+    --sidebar: #f2f4f6;
+}
+
+.dark {
+    color-scheme: dark;
+    --background: #0f172a;
+    --foreground: #f1f5f9;
+    --card: #1e293b;
+    --card-foreground: #f1f5f9;
+    --popover: #1e293b;
+    --popover-foreground: #f1f5f9;
+    --primary: #4ae176;
+    --primary-foreground: #002109;
+    --secondary: #334155;
+    --secondary-foreground: #e2e8f0;
+    --muted: #1e293b;
+    --muted-foreground: #b7c8e1;
+    --accent: #173b2b;
+    --accent-foreground: #6bff8f;
+    --destructive: #fca5a5;
+    --destructive-foreground: #450a0a;
+    --border: #334155;
+    --input: #8293aa;
+    --ring: #4ae176;
+    --brand: #22c55e;
+    --brand-foreground: #002109;
+    --success: #173b2b;
+    --success-foreground: #6bff8f;
+    --warning: #422006;
+    --warning-foreground: #fde68a;
+    --info: #172554;
+    --info-foreground: #bfdbfe;
+    --chart-1: #4ae176;
+    --chart-2: #b7c8e1;
+    --chart-3: #ffb4a9;
+    --chart-4: #fbbf24;
+    --chart-5: #60a5fa;
+    --sidebar-background: #111c30;
+    --sidebar-foreground: #f1f5f9;
+    --sidebar-primary: #4ae176;
+    --sidebar-primary-foreground: #002109;
+    --sidebar-accent: #173b2b;
+    --sidebar-accent-foreground: #6bff8f;
+    --sidebar-border: #334155;
+    --sidebar-ring: #4ae176;
+    --sidebar: #111c30;
+}
+
+html {
+    background-color: var(--background);
+    color: var(--foreground);
+}
diff --git a/resources/js/app.ts b/resources/js/app.ts
index c098682..1048d74 100644
--- a/resources/js/app.ts
+++ b/resources/js/app.ts
@@ -3,7 +3,8 @@ import { resolvePageComponent } from 'laravel-vite-plugin/inertia-helpers';
 import type { DefineComponent } from 'vue';
 import { createApp, h } from 'vue';
 import '../css/app.css';
-import { initializeTheme } from '@/composables/useAppearance';
+import { createAppearance } from '@/composables/useAppearance';
+import { createLocalization } from '@/lib/localization';
 
 const appName = import.meta.env.VITE_APP_NAME || 'Laravel';
 
@@ -17,12 +18,11 @@ createInertiaApp({
     setup({ el, App, props, plugin }) {
         createApp({ render: () => h(App, props) })
             .use(plugin)
+            .use(createLocalization(props.initialPage.props.localization))
+            .use(createAppearance(props.initialPage.props.appearance))
             .mount(el);
     },
     progress: {
-        color: '#4B5563',
+        color: 'var(--primary)',
     },
 });
-
-// This will set light / dark mode on page load...
-initializeTheme();
diff --git a/resources/js/components/AlertError.vue b/resources/js/components/AlertError.vue
index c05937d..3849851 100644
--- a/resources/js/components/AlertError.vue
+++ b/resources/js/components/AlertError.vue
@@ -2,15 +2,15 @@
 import { AlertCircle } from 'lucide-vue-next';
 import { computed } from 'vue';
 import { Alert, AlertDescription, AlertTitle } from '@/components/ui/alert';
+import { useLocalization } from '@/composables/useLocalization';
 
 type Props = {
     errors: string[];
     title?: string;
 };
 
-const props = withDefaults(defineProps<Props>(), {
-    title: 'Something went wrong.',
-});
+const props = defineProps<Props>();
+const { t } = useLocalization();
 
 const uniqueErrors = computed(() => Array.from(new Set(props.errors)));
 </script>
@@ -18,7 +18,7 @@ const uniqueErrors = computed(() => Array.from(new Set(props.errors)));
 <template>
     <Alert variant="destructive">
         <AlertCircle class="size-4" />
-        <AlertTitle>{{ title }}</AlertTitle>
+        <AlertTitle>{{ title ?? t('common.error') }}</AlertTitle>
         <AlertDescription>
             <ul class="list-inside list-disc text-sm">
                 <li v-for="(error, index) in uniqueErrors" :key="index">
diff --git a/resources/js/components/AppearanceTabs.vue b/resources/js/components/AppearanceTabs.vue
index 22712f9..d305c42 100644
--- a/resources/js/components/AppearanceTabs.vue
+++ b/resources/js/components/AppearanceTabs.vue
@@ -1,33 +1,39 @@
 <script setup lang="ts">
 import { Monitor, Moon, Sun } from 'lucide-vue-next';
 import { useAppearance } from '@/composables/useAppearance';
+import { useLocalization } from '@/composables/useLocalization';
 
 const { appearance, updateAppearance } = useAppearance();
+const { t } = useLocalization();
 
 const tabs = [
-    { value: 'light', Icon: Sun, label: 'Light' },
-    { value: 'dark', Icon: Moon, label: 'Dark' },
-    { value: 'system', Icon: Monitor, label: 'System' },
+    { value: 'light', Icon: Sun, label: 'appearance.light' },
+    { value: 'dark', Icon: Moon, label: 'appearance.dark' },
+    { value: 'system', Icon: Monitor, label: 'appearance.system' },
 ] as const;
 </script>
 
 <template>
     <div
-        class="inline-flex gap-1 rounded-lg bg-neutral-100 p-1 dark:bg-neutral-800"
+        role="group"
+        :aria-label="t('appearance.label')"
+        class="border-border bg-muted inline-flex max-w-full gap-1 rounded-lg border p-1"
     >
         <button
             v-for="{ value, Icon, label } in tabs"
             :key="value"
+            type="button"
+            :aria-pressed="appearance === value"
             @click="updateAppearance(value)"
             :class="[
-                'flex items-center rounded-md px-3.5 py-1.5 transition-colors',
+                'min-h-control text-body focus-visible:outline-ring flex min-w-0 items-center justify-center gap-1.5 rounded-md px-3 font-medium transition-colors focus-visible:outline-2 focus-visible:outline-offset-2',
                 appearance === value
-                    ? 'shadow-xs bg-white dark:bg-neutral-700 dark:text-neutral-100'
-                    : 'text-neutral-500 hover:bg-neutral-200/60 hover:text-black dark:text-neutral-400 dark:hover:bg-neutral-700/60',
+                    ? 'bg-primary text-primary-foreground'
+                    : 'text-muted-foreground hover:bg-accent hover:text-accent-foreground',
             ]"
         >
-            <component :is="Icon" class="-ml-1 h-4 w-4" />
-            <span class="ml-1.5 text-sm">{{ label }}</span>
+            <component :is="Icon" class="size-4 shrink-0" aria-hidden="true" />
+            <span>{{ t(label) }}</span>
         </button>
     </div>
 </template>
diff --git a/resources/js/components/ui/button/index.ts b/resources/js/components/ui/button/index.ts
index 26e2c55..7cba0b4 100644
--- a/resources/js/components/ui/button/index.ts
+++ b/resources/js/components/ui/button/index.ts
@@ -11,7 +11,7 @@ export const buttonVariants = cva(
         default:
           "bg-primary text-primary-foreground hover:bg-primary/90",
         destructive:
-          "bg-destructive text-white hover:bg-destructive/90 focus-visible:ring-destructive/20 dark:focus-visible:ring-destructive/40 dark:bg-destructive/60",
+          "bg-destructive text-destructive-foreground hover:bg-destructive/90 focus-visible:ring-destructive/20 dark:focus-visible:ring-destructive/40",
         outline:
           "border bg-background shadow-xs hover:bg-accent hover:text-accent-foreground dark:bg-input/30 dark:border-input dark:hover:bg-input/50",
         secondary:
diff --git a/resources/js/composables/useAppearance.ts b/resources/js/composables/useAppearance.ts
index 0568774..15100c4 100644
--- a/resources/js/composables/useAppearance.ts
+++ b/resources/js/composables/useAppearance.ts
@@ -1,124 +1,107 @@
-import type { ComputedRef, Ref } from 'vue';
-import { computed, onMounted, ref } from 'vue';
+import type { App, ComputedRef, InjectionKey, Ref } from 'vue';
+import { computed, inject, readonly, ref } from 'vue';
 import type { Appearance, ResolvedAppearance } from '@/types';
 
 export type { Appearance, ResolvedAppearance };
 
 export type UseAppearanceReturn = {
-    appearance: Ref<Appearance>;
+    appearance: Readonly<Ref<Appearance>>;
     resolvedAppearance: ComputedRef<ResolvedAppearance>;
     updateAppearance: (value: Appearance) => void;
 };
 
-export function updateTheme(value: Appearance): void {
-    if (typeof window === 'undefined') {
-        return;
-    }
+const appearanceKey: InjectionKey<UseAppearanceReturn> = Symbol('appearance');
 
-    if (value === 'system') {
-        const mediaQueryList = window.matchMedia(
-            '(prefers-color-scheme: dark)',
-        );
-        const systemTheme = mediaQueryList.matches ? 'dark' : 'light';
+export function normalizeAppearance(value: unknown): Appearance {
+    return value === 'light' || value === 'dark' ? value : 'system';
+}
 
+/** Each Vue app owns its state, including each server-rendered request. */
+export function createAppearance(initialAppearance: Appearance = 'system') {
+    const appearance = ref<Appearance>(normalizeAppearance(initialAppearance));
+    const systemDark = ref(false);
+    const resolvedAppearance = computed<ResolvedAppearance>(() =>
+        appearance.value === 'system'
+            ? systemDark.value
+                ? 'dark'
+                : 'light'
+            : appearance.value,
+    );
+
+    function applyTheme(): void {
         document.documentElement.classList.toggle(
             'dark',
-            systemTheme === 'dark',
+            resolvedAppearance.value === 'dark',
         );
-    } else {
-        document.documentElement.classList.toggle('dark', value === 'dark');
-    }
-}
-
-const setCookie = (name: string, value: string, days = 365) => {
-    if (typeof document === 'undefined') {
-        return;
+        document.documentElement.style.colorScheme = resolvedAppearance.value;
+        document.documentElement.dataset.appearance = appearance.value;
     }
 
-    const maxAge = days * 24 * 60 * 60;
-
-    document.cookie = `${name}=${value};path=/;max-age=${maxAge};SameSite=Lax`;
-};
-
-const mediaQuery = () => {
-    if (typeof window === 'undefined') {
-        return null;
-    }
-
-    return window.matchMedia('(prefers-color-scheme: dark)');
-};
-
-const getStoredAppearance = () => {
-    if (typeof window === 'undefined') {
-        return null;
-    }
-
-    return localStorage.getItem('appearance') as Appearance | null;
-};
-
-const prefersDark = (): boolean => {
-    if (typeof window === 'undefined') {
-        return false;
-    }
-
-    return window.matchMedia('(prefers-color-scheme: dark)').matches;
-};
-
-const handleSystemThemeChange = () => {
-    const currentAppearance = getStoredAppearance();
-
-    updateTheme(currentAppearance || 'system');
-};
-
-export function initializeTheme(): void {
-    if (typeof window === 'undefined') {
-        return;
-    }
-
-    // Initialize theme from saved preference or default to system...
-    const savedAppearance = getStoredAppearance();
-    updateTheme(savedAppearance || 'system');
-
-    // Set up system theme change listener...
-    mediaQuery()?.addEventListener('change', handleSystemThemeChange);
-}
-
-const appearance = ref<Appearance>('system');
-
-export function useAppearance(): UseAppearanceReturn {
-    onMounted(() => {
-        const savedAppearance = localStorage.getItem(
-            'appearance',
-        ) as Appearance | null;
-
-        if (savedAppearance) {
-            appearance.value = savedAppearance;
-        }
-    });
-
-    const resolvedAppearance = computed<ResolvedAppearance>(() => {
-        if (appearance.value === 'system') {
-            return prefersDark() ? 'dark' : 'light';
+    function updateAppearance(value: Appearance): void {
+        appearance.value = normalizeAppearance(value);
+        if (typeof window === 'undefined') {
+            return;
         }
 
-        return appearance.value;
-    });
+        applyTheme();
+        try {
+            window.localStorage.setItem('appearance', appearance.value);
+        } catch {
+            // Private or embedded browsers may deny storage; the current theme still works.
+        }
+        document.cookie = `appearance=${appearance.value};path=/;max-age=31536000;SameSite=Lax`;
+    }
 
-    function updateAppearance(value: Appearance) {
-        appearance.value = value;
+    function handleSystemChange(event: MediaQueryListEvent): void {
+        systemDark.value = event.matches;
+        applyTheme();
+    }
 
-        // Store in localStorage for client-side persistence...
-        localStorage.setItem('appearance', value);
-
-        // Store in cookie for SSR...
-        setCookie('appearance', value);
-
-        updateTheme(value);
+    function handleStorageChange(event: StorageEvent): void {
+        if (event.key === 'appearance' || event.key === null) {
+            updateAppearance(normalizeAppearance(event.newValue));
+        }
     }
 
     return {
-        appearance,
+        appearance: readonly(appearance),
         resolvedAppearance,
         updateAppearance,
+        install(app: App): void {
+            app.provide(appearanceKey, {
+                appearance: readonly(appearance),
+                resolvedAppearance,
+                updateAppearance,
+            });
+            if (typeof window === 'undefined') {
+                return;
+            }
+
+            // The blocking head script already reconciled local storage and the SSR cookie.
+            appearance.value = normalizeAppearance(
+                document.documentElement.dataset.appearance ??
+                    initialAppearance,
+            );
+            const media = window.matchMedia('(prefers-color-scheme: dark)');
+            systemDark.value = media.matches;
+            applyTheme();
+            media.addEventListener('change', handleSystemChange);
+            window.addEventListener('storage', handleStorageChange);
+
+            app.onUnmount(() => {
+                media.removeEventListener('change', handleSystemChange);
+                window.removeEventListener('storage', handleStorageChange);
+            });
+        },
     };
 }
+
+export function useAppearance(): UseAppearanceReturn {
+    const appearance = inject(appearanceKey);
+    if (!appearance) {
+        throw new Error(
+            'The appearance plugin must be installed before useAppearance().',
+        );
+    }
+    return appearance;
+}
diff --git a/resources/js/composables/useLocalization.ts b/resources/js/composables/useLocalization.ts
new file mode 100644
index 0000000..d367fc7
--- /dev/null
+++ b/resources/js/composables/useLocalization.ts
@@ -0,0 +1,12 @@
+import { inject } from 'vue';
+import { localizationKey } from '@/lib/localization';
+
+export function useLocalization() {
+    const localization = inject(localizationKey);
+    if (!localization) {
+        throw new Error(
+            'The localization plugin must be installed before useLocalization().',
+        );
+    }
+    return localization;
+}
diff --git a/resources/js/lib/localization.ts b/resources/js/lib/localization.ts
new file mode 100644
index 0000000..6eb212c
--- /dev/null
+++ b/resources/js/lib/localization.ts
@@ -0,0 +1,159 @@
+import { readonly, ref } from 'vue';
+import type { App, InjectionKey } from 'vue';
+import { en } from '@/locales/en';
+import type {
+    LocalizationSettings,
+    TranslationCatalog,
+    TranslationKey,
+    TranslationParameters,
+} from '@/types/localization';
+
+const defaultLocale = 'en';
+const catalogs: Record<string, TranslationCatalog> = { en };
+
+function placeholders(message: string): string[] {
+    return [
+        ...new Set(
+            [...message.matchAll(/\{(\w+)\}/g)].map((match) => match[1]),
+        ),
+    ].sort();
+}
+
+/** Validate every catalog, including messages that no component has rendered yet. */
+export function validateCatalog(catalog: TranslationCatalog): void {
+    for (const key of Object.keys(en) as TranslationKey[]) {
+        const message = Object.hasOwn(catalog, key) ? catalog[key] : undefined;
+        const source = en[key];
+
+        if (message === undefined || typeof message !== typeof source) {
+            throw new Error(`Missing or invalid translation: ${key}`);
+        }
+
+        const expected = placeholders(
+            typeof source === 'string' ? source : source.other,
+        );
+        const forms =
+            typeof message === 'string'
+                ? [message]
+                : Object.values(message ?? {});
+
+        if (
+            typeof message !== 'string' &&
+            (!message || typeof message.other !== 'string')
+        ) {
+            throw new Error(`Missing plural fallback: ${key}`);
+        }
+
+        for (const form of forms) {
+            if (
+                typeof form !== 'string' ||
+                form.trim() === '' ||
+                JSON.stringify(placeholders(form)) !== JSON.stringify(expected)
+            ) {
+                throw new Error(`Invalid translation placeholders: ${key}`);
+            }
+        }
+    }
+}
+
+export function createLocalization(
+    settings: Partial<LocalizationSettings> = {},
+    availableCatalogs: Record<string, TranslationCatalog> = catalogs,
+) {
+    const messages: Record<string, TranslationCatalog> = {
+        ...availableCatalogs,
+        en,
+    };
+    for (const catalog of Object.values(messages)) {
+        validateCatalog(catalog);
+    }
+
+    function resolveLocale(requested: string): string {
+        try {
+            const [canonical] = Intl.getCanonicalLocales(
+                requested.replaceAll('_', '-'),
+            );
+            if (Object.hasOwn(messages, canonical)) {
+                return canonical;
+            }
+            const language = canonical.split('-')[0];
+            return Object.hasOwn(messages, language) ? language : defaultLocale;
+        } catch {
+            return defaultLocale;
+        }
+    }
+
+    const locale = ref(resolveLocale(settings.locale ?? defaultLocale));
+    const timeZone = settings.timeZone ?? 'UTC';
+
+    function number(
+        value: number,
+        options: Intl.NumberFormatOptions = {},
+    ): string {
+        return new Intl.NumberFormat(locale.value, options).format(value);
+    }
+
+    function date(
+        value: Date | number,
+        options: Intl.DateTimeFormatOptions = {},
+    ): string {
+        return new Intl.DateTimeFormat(locale.value, {
+            timeZone,
+            ...options,
+        }).format(value);
+    }
+
+    /** Interpolate plain text only; render with Vue text bindings, never v-html. */
+    function t(
+        key: TranslationKey,
+        parameters: TranslationParameters = {},
+    ): string {
+        const catalog = messages[locale.value];
+        if (!Object.hasOwn(catalog, key)) {
+            throw new Error(`Missing translation: ${key}`);
+        }
+
+        const message = catalog[key];
+        let template: string;
+        if (typeof message === 'string') {
+            template = message;
+        } else {
+            const count = parameters.count;
+            if (typeof count !== 'number' || !Number.isFinite(count)) {
+                throw new Error(`Translation requires a finite count: ${key}`);
+            }
+            template =
+                message[new Intl.PluralRules(locale.value).select(count)] ??
+                message.other;
+        }
+
+        return template.replace(/\{(\w+)\}/g, (_, name: string) => {
+            if (!Object.hasOwn(parameters, name)) {
+                throw new Error(
+                    `Missing translation parameter: ${key}.${name}`,
+                );
+            }
+            const value = parameters[name];
+            return typeof value === 'number' ? number(value) : value;
+        });
+    }
+
+    const localization = {
+        locale: readonly(locale),
+        t,
+        number,
+        date,
+        setLocale(requested: string): void {
+            locale.value = resolveLocale(requested);
+        },
+        install(app: App): void {
+            app.provide(localizationKey, localization);
+        },
+    };
+
+    return localization;
+}
+
+export type Localization = ReturnType<typeof createLocalization>;
+export const localizationKey: InjectionKey<Localization> =
+    Symbol('localization');
diff --git a/resources/js/locales/en.ts b/resources/js/locales/en.ts
new file mode 100644
index 0000000..8b619f4
--- /dev/null
+++ b/resources/js/locales/en.ts
@@ -0,0 +1,15 @@
+export const en = {
+    'appearance.label': 'Color theme',
+    'appearance.light': 'Light',
+    'appearance.dark': 'Dark',
+    'appearance.system': 'System',
+    'common.cancel': 'Cancel',
+    'common.save': 'Save',
+    'common.loading': 'Loading…',
+    'common.error': 'Something went wrong.',
+    'pagination.page': 'Page {page} of {total}',
+    'tickets.count': {
+        one: '{count} ticket',
+        other: '{count} tickets',
+    },
+} as const;
diff --git a/resources/js/ssr.ts b/resources/js/ssr.ts
index cb4a069..f50c59a 100644
--- a/resources/js/ssr.ts
+++ b/resources/js/ssr.ts
@@ -4,6 +4,12 @@ import { resolvePageComponent } from 'laravel-vite-plugin/inertia-helpers';
 import type { DefineComponent } from 'vue';
 import { createSSRApp, h } from 'vue';
 import { renderToString } from 'vue/server-renderer';
+import { createLocalization } from '@/lib/localization';
+import type { LocalizationSettings } from '@/types/localization';
+import {
+    createAppearance,
+    normalizeAppearance,
+} from '@/composables/useAppearance';
 
 const appName = import.meta.env.VITE_APP_NAME || 'Laravel';
 
@@ -19,7 +25,21 @@ createServer(
                     import.meta.glob<DefineComponent>('./pages/**/*.vue'),
                 ),
             setup: ({ App, props, plugin }) =>
-                createSSRApp({ render: () => h(App, props) }).use(plugin),
+                createSSRApp({ render: () => h(App, props) })
+                    .use(plugin)
+                    .use(
+                        createAppearance(
+                            normalizeAppearance(
+                                props.initialPage.props.appearance,
+                            ),
+                        ),
+                    )
+                    .use(
+                        createLocalization(
+                            props.initialPage.props
+                                .localization as LocalizationSettings,
+                        ),
+                    ),
         }),
     { cluster: true },
 );
diff --git a/resources/js/types/global.d.ts b/resources/js/types/global.d.ts
index b4b2319..85fedb6 100644
--- a/resources/js/types/global.d.ts
+++ b/resources/js/types/global.d.ts
@@ -1,4 +1,6 @@
 import type { Auth } from '@/types/auth';
+import type { LocalizationSettings } from '@/types/localization';
+import type { Appearance } from '@/types/ui';
 import type { Page, Router } from '@inertiajs/core';
 import { createHeadManager } from '@inertiajs/vue3';
 
@@ -19,6 +21,8 @@ declare module '@inertiajs/core' {
     export interface InertiaConfig {
         sharedPageProps: {
             name: string;
+            localization: LocalizationSettings;
+            appearance: Appearance;
             auth: Auth;
             sidebarOpen: boolean;
         };
diff --git a/resources/js/types/localization.ts b/resources/js/types/localization.ts
new file mode 100644
index 0000000..0b95926
--- /dev/null
+++ b/resources/js/types/localization.ts
@@ -0,0 +1,13 @@
+import type { en } from '@/locales/en';
+
+export type TranslationKey = keyof typeof en;
+export type PluralMessage = Partial<Record<Intl.LDMLPluralRule, string>> & {
+    other: string;
+};
+export type TranslationCatalog = Record<TranslationKey, string | PluralMessage>;
+export type TranslationParameters = Record<string, string | number>;
+
+export type LocalizationSettings = {
+    locale: string;
+    timeZone: string;
+};
diff --git a/resources/views/app.blade.php b/resources/views/app.blade.php
index 3ef8e7a..56dd700 100644
--- a/resources/views/app.blade.php
+++ b/resources/views/app.blade.php
@@ -1,33 +1,33 @@
 <!DOCTYPE html>
-<html lang="{{ str_replace('_', '-', app()->getLocale()) }}"  @class(['dark' => ($appearance ?? 'system') == 'dark'])>
+<html lang="{{ str_replace('_', '-', app()->getLocale()) }}" data-appearance="{{ $appearance ?? 'system' }}" @class(['dark' => ($appearance ?? 'system') === 'dark'])>
     <head>
         <meta charset="utf-8">
         <meta name="viewport" content="width=device-width, initial-scale=1">
 
-        {{-- Inline script to detect system dark mode preference and apply it immediately --}}
+        {{-- Resolve before first paint; the Vue plugin reads the same data attribute. --}}
         <script>
             (function() {
-                const appearance = '{{ $appearance ?? "system" }}';
-
-                if (appearance === 'system') {
-                    const prefersDark = window.matchMedia('(prefers-color-scheme: dark)').matches;
-
-                    if (prefersDark) {
-                        document.documentElement.classList.add('dark');
+                const root = document.documentElement;
+                let appearance = root.dataset.appearance;
+                try {
+                    const stored = window.localStorage.getItem('appearance');
+                    if (stored !== null) {
+                        appearance = stored;
                     }
+                } catch {
+                    // Fall back to the server cookie when browser storage is unavailable.
                 }
+                appearance = ['light', 'dark', 'system'].includes(appearance) ? appearance : 'system';
+                const dark = appearance === 'dark' || (appearance === 'system' && window.matchMedia('(prefers-color-scheme: dark)').matches);
+                root.dataset.appearance = appearance;
+                root.classList.toggle('dark', dark);
+                root.style.colorScheme = dark ? 'dark' : 'light';
             })();
         </script>
 
-        {{-- Inline style to set the HTML background color based on our theme in app.css --}}
+        {{-- The same tokens are bundled by Vite, so first paint cannot drift. --}}
         <style>
-            html {
-                background-color: oklch(1 0 0);
-            }
-
-            html.dark {
-                background-color: oklch(0.145 0 0);
-            }
+            {!! file_get_contents(resource_path('css/theme.css')) !!}
         </style>
 
         <title inertia>{{ config('app.name', 'Laravel') }}</title>
@@ -36,8 +36,7 @@
         <link rel="icon" href="/favicon.svg" type="image/svg+xml">
         <link rel="apple-touch-icon" href="/apple-touch-icon.png">
 
-        <link rel="preconnect" href="https://fonts.bunny.net">
-        <link href="https://fonts.bunny.net/css?family=instrument-sans:400,500,600" rel="stylesheet" />
+        <link rel="preload" href="{{ asset('fonts/inter/InterVariable.woff2') }}" as="font" type="font/woff2" crossorigin>
 
         @vite(['resources/js/app.ts', "resources/js/pages/{$page['component']}.vue"])
         @inertiaHead
diff --git a/tests/Feature/AppearanceTest.php b/tests/Feature/AppearanceTest.php
new file mode 100644
index 0000000..ed4af50
--- /dev/null
+++ b/tests/Feature/AppearanceTest.php
@@ -0,0 +1,64 @@
+<?php
+
+namespace Tests\Feature;
+
+use Inertia\Testing\AssertableInertia as Assert;
+use PHPUnit\Framework\Attributes\DataProvider;
+use Symfony\Component\Process\Process;
+use Tests\TestCase;
+
+class AppearanceTest extends TestCase
+{
+    #[DataProvider('appearanceCookies')]
+    public function test_initial_render_uses_a_validated_theme(?string $cookie, string $expected): void
+    {
+        if ($cookie !== null) {
+            $this->withUnencryptedCookie('appearance', $cookie);
+        }
+
+        $response = $this->get(route('home'));
+        $response->assertOk()
+            ->assertSee('data-appearance="'.$expected.'"', false)
+            ->assertInertia(fn (Assert $page) => $page->where('appearance', $expected))
+            ->assertDontSee('fonts.bunny.net', false)
+            ->assertDontSee('Instrument Sans', false)
+            ->assertSee('fonts/inter/InterVariable.woff2', false);
+
+        preg_match('/<html[^>]+>/', $response->getContent(), $matches);
+        $this->assertSame($expected === 'dark', str_contains($matches[0], 'class="dark"'));
+        $this->assertStringContainsString(file_get_contents(resource_path('css/theme.css')), $response->getContent());
+    }
+
+    /** @return array<string, array{?string, string}> */
+    public static function appearanceCookies(): array
+    {
+        return [
+            'default' => [null, 'system'],
+            'light' => ['light', 'light'],
+            'dark' => ['dark', 'dark'],
+            'system' => ['system', 'system'],
+            'invalid' => ['invalid', 'system'],
+            'empty' => ['', 'system'],
+            'malformed' => ['</script><script>alert(1)</script>', 'system'],
+        ];
+    }
+
+    public function test_theme_behaviour_and_contrast(): void
+    {
+        $process = new Process(['node', 'tests/Fixtures/appearance.mjs'], base_path());
+        $process->setTimeout(60);
+        $process->run();
+
+        $this->assertTrue($process->isSuccessful(), $process->getOutput().$process->getErrorOutput());
+        $this->assertStringContainsString('Appearance behavior checks passed.', $process->getOutput());
+    }
+
+    public function test_inter_is_bundled_with_its_license(): void
+    {
+        foreach (['InterVariable.woff2', 'InterVariable-Italic.woff2'] as $font) {
+            $this->assertSame('wOF2', file_get_contents(public_path('fonts/inter/'.$font), false, null, 0, 4));
+        }
+
+        $this->assertStringContainsString('SIL OPEN FONT LICENSE', file_get_contents(public_path('fonts/inter/LICENSE.txt')));
+    }
+}
diff --git a/tests/Feature/LocalizationTest.php b/tests/Feature/LocalizationTest.php
new file mode 100644
index 0000000..8727b37
--- /dev/null
+++ b/tests/Feature/LocalizationTest.php
@@ -0,0 +1,47 @@
+<?php
+
+namespace Tests\Feature;
+
+use Illuminate\Support\Facades\Validator;
+use Inertia\Testing\AssertableInertia as Assert;
+use Symfony\Component\Process\Process;
+use Tests\TestCase;
+
+class LocalizationTest extends TestCase
+{
+    public function test_frontend_localization_behaviour(): void
+    {
+        $process = new Process(['node', 'tests/Fixtures/localization.mjs'], base_path());
+        $process->setTimeout(60);
+        $process->run();
+
+        $this->assertTrue($process->isSuccessful(), $process->getOutput().$process->getErrorOutput());
+        $this->assertStringContainsString('Localization behavior checks passed.', $process->getOutput());
+    }
+
+    public function test_initial_page_shares_the_application_locale_and_time_zone(): void
+    {
+        config(['app.timezone' => 'Europe/Berlin']);
+
+        $this->withHeader('Accept-Language', 'de-DE')->get(route('home'))
+            ->assertOk()
+            ->assertSee('lang="en"', false)
+            ->assertInertia(fn (Assert $page) => $page
+                ->where('localization.locale', 'en')
+                ->where('localization.timeZone', 'Europe/Berlin'));
+    }
+
+    public function test_server_validation_remains_english(): void
+    {
+        $this->assertSame('en', config('app.locale'));
+        $this->assertSame('en', config('app.fallback_locale'));
+
+        $validator = Validator::make(['email' => 'invalid'], [
+            'title' => ['required'],
+            'email' => ['email'],
+        ]);
+
+        $this->assertSame('The title field is required.', $validator->errors()->first('title'));
+        $this->assertSame('The email field must be a valid email address.', $validator->errors()->first('email'));
+    }
+}
diff --git a/tests/Fixtures/appearance-preview.mjs b/tests/Fixtures/appearance-preview.mjs
new file mode 100644
index 0000000..1ba16f4
--- /dev/null
+++ b/tests/Fixtures/appearance-preview.mjs
@@ -0,0 +1,47 @@
+import { createApp, h } from 'vue';
+import '../../resources/css/app.css';
+import AppearanceTabs from '../../resources/js/components/AppearanceTabs.vue';
+import AlertError from '../../resources/js/components/AlertError.vue';
+import { Button } from '../../resources/js/components/ui/button';
+import { createAppearance } from '../../resources/js/composables/useAppearance';
+import { createLocalization } from '../../resources/js/lib/localization';
+
+const localization = createLocalization();
+createApp({
+    render: () =>
+        h('main', { class: 'mx-auto grid max-w-xl gap-column p-page' }, [
+            h('h1', { class: 'text-headline' }, 'Emerald design system'),
+            h(AppearanceTabs),
+            h(
+                'section',
+                {
+                    class: 'grid gap-column rounded-md border bg-card p-card text-card-foreground',
+                },
+                [
+                    h('h2', { class: 'text-title' }, 'Readable surfaces'),
+                    h(
+                        'p',
+                        { class: 'text-muted-foreground' },
+                        'Inter, emerald actions and a visible keyboard focus.',
+                    ),
+                    h('label', { for: 'sample-input' }, 'Sample input'),
+                    h('input', {
+                        id: 'sample-input',
+                        class: 'min-h-control rounded-md border border-input bg-background px-3',
+                        placeholder: 'Type here',
+                    }),
+                    h('div', { class: 'flex flex-wrap gap-stack' }, [
+                        h(Button, {}, () => localization.t('common.save')),
+                        h(Button, { variant: 'secondary' }, () =>
+                            localization.t('common.cancel'),
+                        ),
+                        h(Button, { variant: 'destructive' }, () => 'Delete'),
+                    ]),
+                    h(AlertError, { errors: ['A readable validation error.'] }),
+                ],
+            ),
+        ]),
+})
+    .use(localization)
+    .use(createAppearance())
+    .mount('#app');
diff --git a/tests/Fixtures/appearance.html b/tests/Fixtures/appearance.html
new file mode 100644
index 0000000..bfa2bf6
--- /dev/null
+++ b/tests/Fixtures/appearance.html
@@ -0,0 +1,12 @@
+<!doctype html>
+<html lang="en" data-appearance="system">
+    <head>
+        <meta charset="utf-8" />
+        <meta name="viewport" content="width=device-width, initial-scale=1" />
+        <title>Emerald appearance fixture</title>
+    </head>
+    <body>
+        <div id="app"></div>
+        <script type="module" src="./appearance-preview.mjs"></script>
+    </body>
+</html>
diff --git a/tests/Fixtures/appearance.mjs b/tests/Fixtures/appearance.mjs
new file mode 100644
index 0000000..83d7af1
--- /dev/null
+++ b/tests/Fixtures/appearance.mjs
@@ -0,0 +1,219 @@
+import assert from 'node:assert/strict';
+import { readFile } from 'node:fs/promises';
+import { fileURLToPath } from 'node:url';
+import vm from 'node:vm';
+import vue from '@vitejs/plugin-vue';
+import { createServer } from 'vite';
+import { createSSRApp, h } from 'vue';
+import { renderToString } from 'vue/server-renderer';
+
+const server = await createServer({
+    configFile: false,
+    plugins: [vue()],
+    resolve: {
+        alias: {
+            '@': fileURLToPath(new URL('../../resources/js', import.meta.url)),
+        },
+    },
+    server: { middlewareMode: true, hmr: false, watch: null },
+    appType: 'custom',
+});
+
+function browserState(cookie, stored, dark, storageDenied = false) {
+    const root = { dataset: { appearance: cookie }, style: {}, dark: false };
+    root.classList = {
+        toggle: (_, value) => {
+            root.dark = value;
+        },
+    };
+    const listeners = new Map();
+    const storage = new Map(stored === null ? [] : [['appearance', stored]]);
+    const media = {
+        matches: dark,
+        addEventListener: (_, listener) => listeners.set('media', listener),
+        removeEventListener: () => listeners.delete('media'),
+    };
+    return {
+        document: { documentElement: root, cookie: '' },
+        window: {
+            matchMedia: () => media,
+            localStorage: {
+                getItem: (key) => {
+                    if (storageDenied) {
+                        throw new Error('Storage denied');
+                    }
+                    return storage.get(key) ?? null;
+                },
+                setItem: (key, value) => {
+                    if (storageDenied) {
+                        throw new Error('Storage denied');
+                    }
+                    storage.set(key, value);
+                },
+            },
+            addEventListener: (_, listener) =>
+                listeners.set('storage', listener),
+            removeEventListener: () => listeners.delete('storage'),
+        },
+        root,
+        media,
+        listeners,
+        storage,
+    };
+}
+
+try {
+    const { createAppearance, normalizeAppearance } =
+        await server.ssrLoadModule(
+            '/resources/js/composables/useAppearance.ts',
+        );
+    const { createLocalization } = await server.ssrLoadModule(
+        '/resources/js/lib/localization.ts',
+    );
+    const { default: AppearanceTabs } = await server.ssrLoadModule(
+        '/resources/js/components/AppearanceTabs.vue',
+    );
+
+    const renderTheme = (value) =>
+        renderToString(
+            createSSRApp({ render: () => h(AppearanceTabs) })
+                .use(createLocalization())
+                .use(createAppearance(value)),
+        );
+    const [light, dark] = await Promise.all([
+        renderTheme('light'),
+        renderTheme('dark'),
+    ]);
+    const selectedButton = (html) =>
+        html.match(
+            /<button[^>]*aria-pressed="true"[^>]*>([\s\S]*?)<\/button>/,
+        )[1];
+    assert.match(selectedButton(light), />Light</);
+    assert.match(selectedButton(dark), />Dark</);
+    assert.match(light, /role="group" aria-label="Color theme"/);
+    assert.equal((light.match(/type="button"/g) ?? []).length, 3);
+    assert.equal((dark.match(/aria-pressed="true"/g) ?? []).length, 1);
+    assert.equal(normalizeAppearance('unexpected'), 'system');
+
+    const blade = await readFile('resources/views/app.blade.php', 'utf8');
+    const bootstrap = blade.match(/<script>([\s\S]*?)<\/script>/)[1];
+    for (const [cookie, stored, osDark, denied, expected, preference] of [
+        ['system', null, false, false, false, 'system'],
+        ['system', null, true, false, true, 'system'],
+        ['dark', null, false, false, true, 'dark'],
+        ['light', null, true, false, false, 'light'],
+        ['light', 'dark', false, false, true, 'dark'],
+        ['dark', null, false, true, true, 'dark'],
+        ['light', 'invalid', true, false, true, 'system'],
+    ]) {
+        const state = browserState(cookie, stored, osDark, denied);
+        vm.runInNewContext(bootstrap, state);
+        assert.equal(state.root.dark, expected);
+        assert.equal(state.root.dataset.appearance, preference);
+        assert.equal(state.root.style.colorScheme, expected ? 'dark' : 'light');
+    }
+
+    for (const denied of [false, true]) {
+        const state = browserState('system', null, false, denied);
+        globalThis.window = state.window;
+        globalThis.document = state.document;
+        const theme = createAppearance();
+        let cleanup;
+        theme.install({
+            provide() {},
+            onUnmount(callback) {
+                cleanup = callback;
+            },
+        });
+        assert.equal(theme.resolvedAppearance.value, 'light');
+        state.listeners.get('media')({ matches: true });
+        assert.equal(theme.resolvedAppearance.value, 'dark');
+        assert.equal(state.root.dark, true);
+        theme.updateAppearance('light');
+        assert.equal(state.root.dark, false);
+        assert.match(state.document.cookie, /appearance=light;.*SameSite=Lax/);
+        state.listeners.get('media')({ matches: false });
+        state.listeners.get('media')({ matches: true });
+        assert.equal(theme.resolvedAppearance.value, 'light');
+        theme.updateAppearance('system');
+        assert.equal(theme.resolvedAppearance.value, 'dark');
+        state.listeners.get('storage')({
+            key: 'appearance',
+            newValue: 'light',
+        });
+        assert.equal(theme.appearance.value, 'light');
+        state.listeners.get('storage')({ key: null, newValue: null });
+        assert.equal(theme.appearance.value, 'system');
+        if (!denied) {
+            assert.equal(state.storage.get('appearance'), 'system');
+        }
+        cleanup();
+        assert.equal(state.listeners.size, 0);
+        delete globalThis.window;
+        delete globalThis.document;
+    }
+
+    const css = await readFile('resources/css/theme.css', 'utf8');
+    function luminance(hex) {
+        const values = hex
+            .match(/[a-f\d]{2}/gi)
+            .map((part) => parseInt(part, 16) / 255)
+            .map((value) =>
+                value <= 0.04045
+                    ? value / 12.92
+                    : ((value + 0.055) / 1.055) ** 2.4,
+            );
+        return values[0] * 0.2126 + values[1] * 0.7152 + values[2] * 0.0722;
+    }
+    function contrast(first, second) {
+        const values = [luminance(first), luminance(second)].sort(
+            (a, b) => b - a,
+        );
+        return (values[0] + 0.05) / (values[1] + 0.05);
+    }
+    for (const selector of [':root', '.dark']) {
+        const declarations = css
+            .slice(css.indexOf(selector))
+            .match(/\{([^}]+)\}/)[1];
+        const tokens = Object.fromEntries(
+            [...declarations.matchAll(/--([\w-]+):\s*(#[a-f\d]{6})/gi)].map(
+                (match) => [match[1], match[2]],
+            ),
+        );
+        for (const [background, foreground] of [
+            ['background', 'foreground'],
+            ['card', 'card-foreground'],
+            ['popover', 'popover-foreground'],
+            ['primary', 'primary-foreground'],
+            ['secondary', 'secondary-foreground'],
+            ['muted', 'muted-foreground'],
+            ['accent', 'accent-foreground'],
+            ['destructive', 'destructive-foreground'],
+            ['card', 'destructive'],
+            ['brand', 'brand-foreground'],
+            ['success', 'success-foreground'],
+            ['warning', 'warning-foreground'],
+            ['info', 'info-foreground'],
+            ['sidebar-background', 'sidebar-foreground'],
+            ['sidebar-accent', 'sidebar-accent-foreground'],
+            ['sidebar-primary', 'sidebar-primary-foreground'],
+        ]) {
+            assert.ok(
+                contrast(tokens[background], tokens[foreground]) >= 4.5,
+                selector + ': ' + background + '/' + foreground,
+            );
+        }
+        assert.ok(contrast(tokens.background, tokens.ring) >= 3);
+        assert.ok(contrast(tokens.card, tokens.input) >= 3);
+    }
+    const appCss = await readFile('resources/css/app.css', 'utf8');
+    assert.match(appCss, /@media \(prefers-reduced-motion: reduce\)/);
+    assert.match(appCss, /animation-duration: 0.01ms !important/);
+    assert.match(appCss, /transition-duration: 0.01ms !important/);
+    assert.match(appCss, /scroll-behavior: auto !important/);
+    console.log('Appearance behavior checks passed.');
+} finally {
+    delete globalThis.window;
+    delete globalThis.document;
+    await server.close();
+}
diff --git a/tests/Fixtures/localization.mjs b/tests/Fixtures/localization.mjs
new file mode 100644
index 0000000..e560de9
--- /dev/null
+++ b/tests/Fixtures/localization.mjs
@@ -0,0 +1,169 @@
+import assert from 'node:assert/strict';
+import { fileURLToPath } from 'node:url';
+import vue from '@vitejs/plugin-vue';
+import { createServer } from 'vite';
+import { computed, createSSRApp, h } from 'vue';
+import { renderToString } from 'vue/server-renderer';
+
+const server = await createServer({
+    configFile: false,
+    plugins: [vue()],
+    resolve: {
+        alias: {
+            '@': fileURLToPath(new URL('../../resources/js', import.meta.url)),
+        },
+    },
+    server: { middlewareMode: true, hmr: false, watch: null },
+    appType: 'custom',
+});
+
+try {
+    const { createLocalization, validateCatalog } = await server.ssrLoadModule(
+        '/resources/js/lib/localization.ts',
+    );
+    const { useLocalization } = await server.ssrLoadModule(
+        '/resources/js/composables/useLocalization.ts',
+    );
+    const { en } = await server.ssrLoadModule('/resources/js/locales/en.ts');
+    const { default: AlertError } = await server.ssrLoadModule(
+        '/resources/js/components/AlertError.vue',
+    );
+
+    const english = createLocalization();
+    assert.equal(english.t('common.save'), 'Save');
+    assert.equal(
+        english.t('pagination.page', { page: 0, total: 2000 }),
+        'Page 0 of 2,000',
+    );
+    for (const [count, expected] of [
+        [0, '0 tickets'],
+        [1, '1 ticket'],
+        [2, '2 tickets'],
+        [1.5, '1.5 tickets'],
+    ]) {
+        assert.equal(english.t('tickets.count', { count }), expected);
+    }
+    assert.equal(
+        english.number(1234.5, { minimumFractionDigits: 2 }),
+        '1,234.50',
+    );
+    assert.equal(
+        english.number(12, { style: 'currency', currency: 'USD' }),
+        '$12.00',
+    );
+    assert.equal(english.date(new Date('2026-09-09T23:30:00Z')), '9/9/2026');
+    assert.equal(
+        createLocalization({ timeZone: 'Europe/Berlin' }).date(
+            new Date('2026-09-09T23:30:00Z'),
+        ),
+        '9/10/2026',
+    );
+    assert.throws(() => english.t('missing.key'), /Missing translation/);
+    assert.throws(() => english.t('toString'), /Missing translation/);
+    assert.throws(
+        () => english.t('pagination.page', { page: 1 }),
+        /Missing translation parameter/,
+    );
+    for (const count of [undefined, '1', Infinity, NaN]) {
+        assert.throws(
+            () => english.t('tickets.count', { count }),
+            /finite count/,
+        );
+    }
+
+    const incomplete = { ...en };
+    delete incomplete['common.save'];
+    assert.throws(
+        () => validateCatalog(incomplete),
+        /Missing or invalid translation/,
+    );
+    assert.throws(
+        () =>
+            validateCatalog({
+                ...en,
+                'tickets.count': { one: '{count} ticket' },
+            }),
+        /plural fallback/,
+    );
+    assert.throws(
+        () => validateCatalog({ ...en, 'pagination.page': 'Page {unknown}' }),
+        /placeholders/,
+    );
+    assert.throws(
+        () => validateCatalog({ ...en, 'common.save': '' }),
+        /placeholders/,
+    );
+    assert.throws(
+        () => createLocalization({}, { de: incomplete }),
+        /Missing or invalid translation/,
+    );
+
+    for (const locale of ['de-DE', 'en_US', 'not a locale', '']) {
+        assert.equal(createLocalization({ locale }).locale.value, 'en');
+    }
+
+    const de = {
+        ...en,
+        'common.error': 'Etwas ist schiefgelaufen.',
+        'common.save': 'Speichern',
+    };
+    const translated = createLocalization({ locale: 'de_DE' }, { de });
+    assert.equal(translated.locale.value, 'de');
+    assert.equal(translated.number(1234.5), '1.234,5');
+    const label = computed(() => translated.t('common.save'));
+    assert.equal(label.value, 'Speichern');
+    translated.setLocale('en');
+    assert.equal(label.value, 'Save');
+    translated.setLocale('de');
+    assert.equal(label.value, 'Speichern');
+    assert.equal(english.locale.value, 'en');
+
+    const renderAlert = (localization, props = {}) =>
+        renderToString(
+            createSSRApp({
+                render: () =>
+                    h(AlertError, {
+                        errors: ['Invalid input', 'Invalid input'],
+                        ...props,
+                    }),
+            }).use(localization),
+        );
+    const [englishHtml, germanHtml] = await Promise.all([
+        renderAlert(english),
+        renderAlert(translated),
+    ]);
+    assert.match(englishHtml, /Something went wrong\./);
+    assert.match(germanHtml, /Etwas ist schiefgelaufen\./);
+    assert.equal((englishHtml.match(/Invalid input/g) ?? []).length, 1);
+    assert.match(
+        await renderAlert(translated, { title: 'Custom title' }),
+        /Custom title/,
+    );
+    assert.doesNotMatch(
+        await renderAlert(translated, { title: '' }),
+        /Etwas ist schiefgelaufen/,
+    );
+
+    const InterpolatedText = {
+        setup() {
+            const { t } = useLocalization();
+            return () =>
+                h(
+                    'p',
+                    t('pagination.page', {
+                        page: '<script>alert(1)</script>',
+                        total: 1,
+                    }),
+                );
+        },
+    };
+    const escaped = await renderToString(
+        createSSRApp(InterpolatedText).use(createLocalization()),
+    );
+    assert.match(escaped, /&lt;script&gt;/);
+    assert.doesNotMatch(escaped, /<script>/);
+
+    console.log('Localization behavior checks passed.');
+} finally {
+    await server.close();
+}