apps/portal-shell/src/app/app.scss

@@ -21,6 +21,12 @@
   min-width: 0;
   overflow-y: auto;
   background-color: #f9fafb;
+  color: #111827;
+
+  :where(.dark) & {
+    background-color: #0b1220;
+    color: #e5e7eb;
+  }
 }
 
 // Skip-link (WCAG 2.4.1 "Bypass Blocks"). Hidden by default, fully

apps/portal-shell/src/app/components/header/header.html

@@ -1,12 +1,14 @@
-<header class="flex h-16 items-center border-b border-gray-200 bg-white">
+<header
+  class="flex h-16 items-center border-b border-gray-200 bg-white dark:border-gray-800 dark:bg-gray-900"
+>
   <div class="header__logo-zone" [class.header__logo-zone--collapsed]="collapsed()">
     <a
       routerLink="/"
-      class="flex h-full items-center gap-3 rounded-md text-brand-primary-500 transition-colors hover:text-brand-primary-600 focus-visible:outline-2 focus-visible:outline-offset-4 focus-visible:outline-brand-primary-500"
+      class="flex h-full items-center gap-3 rounded-md text-brand-primary-500 transition-colors hover:text-brand-primary-600 focus-visible:outline-2 focus-visible:outline-offset-4 focus-visible:outline-brand-primary-500 dark:text-brand-primary-300 dark:hover:text-brand-primary-200"
       [class.justify-center]="collapsed()"
     >
       <img
-        src="logos/apf-portal.svg"
+        src="logos/apf-small.png"
         alt=""
         aria-hidden="true"
         width="36"
@@ -23,14 +25,14 @@
     <form role="search" class="mx-2 hidden flex-1 md:flex" (submit)="$event.preventDefault()">
       <label class="sr-only" for="global-search">Search the portal</label>
       <div
-        class="flex w-full max-w-xl items-center gap-2 rounded-full border border-gray-200 bg-gray-50 px-4 py-2 focus-within:border-brand-primary-500 focus-within:bg-white focus-within:ring-2 focus-within:ring-brand-primary-200"
+        class="flex w-full max-w-xl items-center gap-2 rounded-full border border-gray-200 bg-gray-50 px-4 py-2 focus-within:border-brand-primary-500 focus-within:bg-white focus-within:ring-2 focus-within:ring-brand-primary-200 dark:border-gray-700 dark:bg-gray-800 dark:focus-within:border-brand-primary-300 dark:focus-within:bg-gray-900 dark:focus-within:ring-brand-primary-700"
       >
         <app-icon name="search" [size]="18" />
         <input
           id="global-search"
           type="search"
           placeholder="Search…"
-          class="w-full bg-transparent text-sm text-gray-800 placeholder:text-gray-400 focus:outline-none"
+          class="w-full bg-transparent text-sm text-gray-800 placeholder:text-gray-400 focus:outline-none dark:text-gray-100 dark:placeholder:text-gray-500"
         />
       </div>
     </form>
@@ -38,25 +40,26 @@
     <nav aria-label="Primary" class="ml-auto flex items-center gap-1">
       <button
         type="button"
-        class="relative inline-flex h-11 w-11 items-center justify-center rounded-full text-gray-600 transition-colors hover:bg-gray-100 hover:text-brand-primary-500 focus-visible:outline-2 focus-visible:outline-offset-2 focus-visible:outline-brand-primary-500"
+        class="relative inline-flex h-11 w-11 items-center justify-center rounded-full text-gray-600 transition-colors hover:bg-gray-100 hover:text-brand-primary-500 focus-visible:outline-2 focus-visible:outline-offset-2 focus-visible:outline-brand-primary-500 dark:text-gray-300 dark:hover:bg-gray-800 dark:hover:text-brand-primary-300"
         aria-label="Notifications"
       >
         <app-icon name="bell" />
         <span
           aria-hidden="true"
-          class="absolute right-2 top-2 h-2 w-2 rounded-full bg-brand-accent-400 ring-2 ring-white"
+          class="absolute right-2 top-2 h-2 w-2 rounded-full bg-brand-accent-400 ring-2 ring-white dark:ring-gray-900"
         ></span>
       </button>
       <button
         type="button"
-        class="inline-flex h-11 w-11 items-center justify-center rounded-full text-gray-600 transition-colors hover:bg-gray-100 hover:text-brand-primary-500 focus-visible:outline-2 focus-visible:outline-offset-2 focus-visible:outline-brand-primary-500"
+        class="inline-flex h-11 w-11 items-center justify-center rounded-full text-gray-600 transition-colors hover:bg-gray-100 hover:text-brand-primary-500 focus-visible:outline-2 focus-visible:outline-offset-2 focus-visible:outline-brand-primary-500 dark:text-gray-300 dark:hover:bg-gray-800 dark:hover:text-brand-primary-300"
         aria-label="Help"
       >
         <app-icon name="circle-help" />
       </button>
+      <app-theme-switcher />
       <button
         type="button"
-        class="inline-flex h-11 w-11 items-center justify-center rounded-full text-gray-600 transition-colors hover:bg-gray-100 hover:text-brand-primary-500 focus-visible:outline-2 focus-visible:outline-offset-2 focus-visible:outline-brand-primary-500"
+        class="inline-flex h-11 w-11 items-center justify-center rounded-full text-gray-600 transition-colors hover:bg-gray-100 hover:text-brand-primary-500 focus-visible:outline-2 focus-visible:outline-offset-2 focus-visible:outline-brand-primary-500 dark:text-gray-300 dark:hover:bg-gray-800 dark:hover:text-brand-primary-300"
         aria-label="Settings"
       >
         <app-icon name="settings" />

apps/portal-shell/src/app/components/header/header.scss

@@ -15,6 +15,10 @@
   border-right: 1px solid #e5e7eb;
   transition: width 0.18s ease-out;
 
+  :where(.dark) & {
+    border-right-color: #1f2937;
+  }
+
   &--collapsed {
     width: 4rem;
     padding: 0 0.5rem;

apps/portal-shell/src/app/components/header/header.spec.ts

@@ -48,6 +48,12 @@ describe('Header', () => {
     expect(root.querySelector('button[aria-label="Settings"]')).not.toBeNull();
   });
 
+  it('embeds the theme switcher', async () => {
+    const fixture = TestBed.createComponent(Header);
+    await fixture.whenStable();
+    expect(fixture.nativeElement.querySelector('app-theme-switcher')).not.toBeNull();
+  });
+
   it('renders the logo zone, expanded with wordmark by default', async () => {
     const fixture = TestBed.createComponent(Header);
     fixture.detectChanges();

apps/portal-shell/src/app/components/header/header.ts

@@ -1,6 +1,7 @@
 import { ChangeDetectionStrategy, Component, inject } from '@angular/core';
 import { RouterLink } from '@angular/router';
 import { Icon } from '../icon/icon';
+import { ThemeSwitcher } from '../theme-switcher/theme-switcher';
 import { LayoutStateService } from '../../state/layout-state.service';
 
 /**
@@ -23,7 +24,7 @@ import { LayoutStateService } from '../../state/layout-state.service';
  */
 @Component({
   selector: 'app-header',
-  imports: [RouterLink, Icon],
+  imports: [RouterLink, Icon, ThemeSwitcher],
   templateUrl: './header.html',
   styleUrl: './header.scss',
   changeDetection: ChangeDetectionStrategy.OnPush,

apps/portal-shell/src/app/components/icon/icon.ts

@@ -4,6 +4,8 @@ import {
   Bell,
   Building2,
   Calendar,
+  Check,
+  ChevronDown,
   ChevronLeft,
   ChevronRight,
   CircleHelp,
@@ -15,9 +17,12 @@ import {
   type LucideIconData,
   LucideAngularModule,
   Mail,
+  Monitor,
+  Moon,
   Search,
   Settings,
   Store,
+  Sun,
   UsersRound,
   Wrench,
 } from 'lucide-angular';
@@ -48,6 +53,8 @@ const ICON_REGISTRY = {
   bell: Bell,
   'building-2': Building2,
   calendar: Calendar,
+  check: Check,
+  'chevron-down': ChevronDown,
   'chevron-left': ChevronLeft,
   'chevron-right': ChevronRight,
   'circle-help': CircleHelp,
@@ -57,9 +64,12 @@ const ICON_REGISTRY = {
   home: House,
   'layout-dashboard': LayoutDashboard,
   mail: Mail,
+  monitor: Monitor,
+  moon: Moon,
   search: Search,
   settings: Settings,
   store: Store,
+  sun: Sun,
   'users-round': UsersRound,
   wrench: Wrench,
 } satisfies Record<string, LucideIconData>;

apps/portal-shell/src/app/components/sidebar/sidebar.html

@@ -1,5 +1,5 @@
 <aside
-  class="sidebar flex h-full flex-col border-r border-gray-200 bg-white"
+  class="sidebar flex h-full flex-col border-r border-gray-200 bg-white dark:border-gray-800 dark:bg-gray-900"
   [class.sidebar--collapsed]="collapsed()"
   [attr.aria-label]="'Sidebar navigation'"
 >
@@ -7,7 +7,9 @@
     @for (group of menu; track group.label) {
     <div class="mb-4">
       @if (!collapsed()) {
-      <h2 class="px-3 pb-1 text-xs font-semibold uppercase tracking-wide text-gray-500">
+      <h2
+        class="px-3 pb-1 text-xs font-semibold uppercase tracking-wide text-gray-500 dark:text-gray-400"
+      >
         {{ group.label }}
       </h2>
       }
@@ -49,7 +51,7 @@
     }
   </nav>
 
-  <div class="border-t border-gray-200 px-2 py-3">
+  <div class="border-t border-gray-200 px-2 py-3 dark:border-gray-800">
     <nav aria-label="Accessibility" class="mb-3">
       <ul class="flex flex-col gap-1">
         <li>
@@ -86,8 +88,12 @@
     </nav>
 
     @if (!collapsed()) {
-    <p class="mb-2 px-3 text-xs text-gray-500" aria-label="Current role: anonymous (signed-out)">
+    <p
-      Role: <span class="font-medium text-gray-700">Anonymous</span>
+      class="mb-2 px-3 text-xs text-gray-500 dark:text-gray-400"
+      aria-label="Current role: anonymous (signed-out)"
+    >
+      Role:
+      <span class="font-medium text-gray-700 dark:text-gray-200">Anonymous</span>
     </p>
     }
 

apps/portal-shell/src/app/components/sidebar/sidebar.scss

@@ -44,14 +44,27 @@
     background-color 0.12s ease-out,
     color 0.12s ease-out;
 
+  :where(.dark) & {
+    color: #e5e7eb;
+  }
+
   &:hover {
     background-color: #f3f4f6;
     color: var(--color-brand-primary-500);
+
+    :where(.dark) & {
+      background-color: #1f2937;
+      color: var(--color-brand-primary-300);
+    }
   }
 
   &:focus-visible {
     outline: 2px solid var(--color-brand-primary-500);
     outline-offset: 2px;
+
+    :where(.dark) & {
+      outline-color: var(--color-brand-primary-300);
+    }
   }
 }
 
@@ -63,6 +76,15 @@
     background-color: var(--color-brand-primary-600);
     color: #fff;
   }
+
+  :where(.dark) & {
+    background-color: var(--color-brand-primary-400);
+
+    &:hover {
+      background-color: var(--color-brand-primary-300);
+      color: #0b1620;
+    }
+  }
 }
 
 .sidebar__label {

apps/portal-shell/src/app/components/theme-switcher/theme-switcher.html

@@ -0,0 +1,30 @@
+<button
+  type="button"
+  class="inline-flex h-11 w-11 items-center justify-center rounded-full text-gray-600 transition-colors hover:bg-gray-100 hover:text-brand-primary-500 focus-visible:outline-2 focus-visible:outline-offset-2 focus-visible:outline-brand-primary-500 dark:text-gray-300 dark:hover:bg-gray-800 dark:hover:text-brand-primary-300"
+  [cdkMenuTriggerFor]="themeMenu"
+  [attr.aria-label]="'Theme: ' + currentLabel() + ' (open menu)'"
+>
+  <app-icon [name]="currentIcon()" />
+</button>
+
+<ng-template #themeMenu>
+  <div cdkMenu class="theme-switcher__menu" role="menu" aria-label="Theme">
+    @for (option of options; track option.mode) {
+    <button
+      type="button"
+      cdkMenuItem
+      class="theme-switcher__item"
+      [class.theme-switcher__item--active]="mode() === option.mode"
+      [attr.aria-checked]="mode() === option.mode"
+      role="menuitemradio"
+      (click)="select(option.mode)"
+    >
+      <app-icon [name]="option.icon" />
+      <span class="theme-switcher__label">{{ option.label }}</span>
+      @if (mode() === option.mode) {
+      <app-icon name="check" [size]="16" />
+      }
+    </button>
+    }
+  </div>
+</ng-template>

apps/portal-shell/src/app/components/theme-switcher/theme-switcher.scss

@@ -0,0 +1,72 @@
+// CDK Menu ships no opinions on visual styling — we colour, frame and
+// align it here. The width pin keeps the three items aligned even
+// when the labels grow under translation. dark-mode surface mirrors
+// the rest of the shell (gray-900 background, gray-800 border).
+
+.theme-switcher__menu {
+  min-width: 11rem;
+  margin-top: 0.25rem;
+  padding: 0.25rem;
+  background-color: #fff;
+  border: 1px solid #e5e7eb;
+  border-radius: 0.5rem;
+  box-shadow:
+    0 4px 6px -1px rgba(0, 0, 0, 0.1),
+    0 2px 4px -2px rgba(0, 0, 0, 0.05);
+
+  :where(.dark) & {
+    background-color: #111827;
+    border-color: #1f2937;
+  }
+}
+
+.theme-switcher__item {
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
+  width: 100%;
+  min-height: 2.5rem;
+  padding: 0.5rem 0.75rem;
+  border-radius: 0.375rem;
+  background: transparent;
+  border: 0;
+  color: #1f2937;
+  font-size: 0.875rem;
+  text-align: left;
+  cursor: pointer;
+  transition:
+    background-color 0.12s ease-out,
+    color 0.12s ease-out;
+
+  :where(.dark) & {
+    color: #f3f4f6;
+  }
+
+  &:hover {
+    background-color: #f3f4f6;
+    color: var(--color-brand-primary-500);
+
+    :where(.dark) & {
+      background-color: #1f2937;
+      color: var(--color-brand-primary-300);
+    }
+  }
+
+  &:focus-visible {
+    outline: 2px solid var(--color-brand-primary-500);
+    outline-offset: -2px;
+  }
+}
+
+.theme-switcher__label {
+  flex: 1 1 auto;
+}
+
+.theme-switcher__item--active {
+  font-weight: 600;
+  color: var(--color-brand-primary-500);
+
+  :where(.dark) & {
+    color: var(--color-brand-primary-300);
+  }
+}

apps/portal-shell/src/app/components/theme-switcher/theme-switcher.spec.ts

@@ -0,0 +1,37 @@
+import { TestBed } from '@angular/core/testing';
+import { LayoutStateService } from '../../state/layout-state.service';
+import { ThemeSwitcher } from './theme-switcher';
+
+describe('ThemeSwitcher', () => {
+  beforeEach(async () => {
+    localStorage.clear();
+    document.documentElement.classList.remove('dark');
+    await TestBed.configureTestingModule({ imports: [ThemeSwitcher] }).compileComponents();
+  });
+
+  it('renders a trigger button announcing the current mode', async () => {
+    const fixture = TestBed.createComponent(ThemeSwitcher);
+    await fixture.whenStable();
+    const button = (fixture.nativeElement as HTMLElement).querySelector(
+      'button[aria-haspopup="menu"]',
+    ) as HTMLButtonElement | null;
+    expect(button).not.toBeNull();
+    expect(button?.getAttribute('aria-label')).toContain('System');
+  });
+
+  it('updates the trigger label and effective theme when a mode is selected', async () => {
+    const layout = TestBed.inject(LayoutStateService);
+    const fixture = TestBed.createComponent(ThemeSwitcher);
+    fixture.detectChanges();
+    await fixture.whenStable();
+
+    layout.setThemeMode('dark');
+    fixture.detectChanges();
+    await fixture.whenStable();
+    const button = (fixture.nativeElement as HTMLElement).querySelector(
+      'button[aria-haspopup="menu"]',
+    ) as HTMLButtonElement | null;
+    expect(button?.getAttribute('aria-label')).toContain('Dark');
+    expect(document.documentElement.classList.contains('dark')).toBe(true);
+  });
+});

apps/portal-shell/src/app/components/theme-switcher/theme-switcher.ts

@@ -0,0 +1,55 @@
+import { CdkMenu, CdkMenuItem, CdkMenuTrigger } from '@angular/cdk/menu';
+import { ChangeDetectionStrategy, Component, computed, inject } from '@angular/core';
+import { Icon, type IconName } from '../icon/icon';
+import { LayoutStateService, type ThemeMode } from '../../state/layout-state.service';
+
+/**
+ * Light / dark / auto color-scheme switcher rendered in the header.
+ *
+ * The trigger is a single icon button whose glyph reflects the
+ * currently SELECTED mode (sun / moon / monitor — not the effective
+ * theme), so the user can tell which mode they're in even when `auto`
+ * happens to resolve to the same scheme as a manual pick. Clicking
+ * the button opens a CDK-driven menu (proper roving tabindex,
+ * Escape/click-outside dismissal) with the three options; the
+ * currently-selected one gets a check mark.
+ *
+ * State lives in `LayoutStateService` — this component is a thin view
+ * over `themeMode()` + `setThemeMode()`.
+ */
+
+interface ModeOption {
+  readonly mode: ThemeMode;
+  readonly label: string;
+  readonly icon: IconName;
+}
+
+const MODE_OPTIONS: readonly ModeOption[] = [
+  { mode: 'light', label: 'Light', icon: 'sun' },
+  { mode: 'dark', label: 'Dark', icon: 'moon' },
+  { mode: 'auto', label: 'System', icon: 'monitor' },
+];
+
+@Component({
+  selector: 'app-theme-switcher',
+  imports: [CdkMenu, CdkMenuItem, CdkMenuTrigger, Icon],
+  templateUrl: './theme-switcher.html',
+  styleUrl: './theme-switcher.scss',
+  changeDetection: ChangeDetectionStrategy.OnPush,
+})
+export class ThemeSwitcher {
+  private readonly layout = inject(LayoutStateService);
+
+  protected readonly options = MODE_OPTIONS;
+  protected readonly mode = this.layout.themeMode;
+  protected readonly currentIcon = computed<IconName>(
+    () => MODE_OPTIONS.find((o) => o.mode === this.mode())?.icon ?? 'monitor',
+  );
+  protected readonly currentLabel = computed(
+    () => MODE_OPTIONS.find((o) => o.mode === this.mode())?.label ?? 'System',
+  );
+
+  protected select(mode: ThemeMode): void {
+    this.layout.setThemeMode(mode);
+  }
+}

apps/portal-shell/src/app/pages/accessibility/accessibility.html

@@ -1,18 +1,26 @@
 <article class="mx-auto max-w-3xl px-6 py-12" [attr.lang]="copy().htmlLang">
-  <h1 class="text-3xl font-semibold tracking-tight text-gray-900">{{ copy().pageTitle }}</h1>
+  <h1 class="text-3xl font-semibold tracking-tight text-gray-900 dark:text-gray-100">
+    {{ copy().pageTitle }}
+  </h1>
 
   <section class="mt-8" aria-labelledby="intro-heading">
-    <h2 id="intro-heading" class="text-lg font-medium text-gray-900">{{ copy().introTitle }}</h2>
+    <h2 id="intro-heading" class="text-lg font-medium text-gray-900 dark:text-gray-100">
-    <p class="mt-3 text-base leading-relaxed text-gray-700">{{ copy().introBody }}</p>
+      {{ copy().introTitle }}
+    </h2>
+    <p class="mt-3 text-base leading-relaxed text-gray-700 dark:text-gray-300">
+      {{ copy().introBody }}
+    </p>
   </section>
 
   <section
-    class="mt-8 rounded-md border-l-4 border-amber-400 bg-amber-50 p-4"
+    class="mt-8 rounded-md border-l-4 border-amber-400 bg-amber-50 p-4 dark:border-amber-500 dark:bg-amber-950/40"
     aria-labelledby="status-heading"
   >
-    <h2 id="status-heading" class="text-base font-medium text-amber-900">
+    <h2 id="status-heading" class="text-base font-medium text-amber-900 dark:text-amber-200">
       {{ copy().panelNoticeTitle }}
     </h2>
-    <p class="mt-2 text-sm leading-relaxed text-amber-900">{{ copy().panelNoticeBody }}</p>
+    <p class="mt-2 text-sm leading-relaxed text-amber-900 dark:text-amber-200">
+      {{ copy().panelNoticeBody }}
+    </p>
   </section>
 </article>

apps/portal-shell/src/app/pages/home/home.html

@@ -1,30 +1,36 @@
 <section class="mx-auto max-w-3xl px-6 py-12">
-  <h1 class="text-3xl font-semibold tracking-tight text-gray-900">Welcome to APF Portal</h1>
+  <h1 class="text-3xl font-semibold tracking-tight text-gray-900 dark:text-gray-100">
-  <p class="mt-4 text-base leading-relaxed text-gray-700">
+    Welcome to APF Portal
+  </h1>
+  <p class="mt-4 text-base leading-relaxed text-gray-700 dark:text-gray-300">
     This is the placeholder home page for the APF Portal. Real content lands once the comms team and
     the accessibility review panel sign off on the copy.
   </p>
 
   <section
     aria-labelledby="system-status-heading"
-    class="mt-10 rounded-md border border-gray-200 bg-white p-5 shadow-sm"
+    class="mt-10 rounded-md border border-gray-200 bg-white p-5 shadow-sm dark:border-gray-800 dark:bg-gray-900"
   >
-    <h2 id="system-status-heading" class="text-base font-medium text-gray-900">System status</h2>
+    <h2 id="system-status-heading" class="text-base font-medium text-gray-900 dark:text-gray-100">
+      System status
+    </h2>
     @if (isLoading()) {
-    <p class="mt-2 text-sm text-gray-600" aria-live="polite">Checking the backend…</p>
+    <p class="mt-2 text-sm text-gray-600 dark:text-gray-400" aria-live="polite">
+      Checking the backend…
+    </p>
     } @else if (health(); as h) {
     <dl class="mt-3 grid grid-cols-2 gap-x-4 gap-y-2 text-sm">
-      <dt class="text-gray-500">Service</dt>
+      <dt class="text-gray-500 dark:text-gray-400">Service</dt>
-      <dd class="font-medium text-gray-900">{{ h.service }}</dd>
+      <dd class="font-medium text-gray-900 dark:text-gray-100">{{ h.service }}</dd>
-      <dt class="text-gray-500">Version</dt>
+      <dt class="text-gray-500 dark:text-gray-400">Version</dt>
-      <dd class="font-medium text-gray-900">{{ h.version }}</dd>
+      <dd class="font-medium text-gray-900 dark:text-gray-100">{{ h.version }}</dd>
-      <dt class="text-gray-500">Uptime</dt>
+      <dt class="text-gray-500 dark:text-gray-400">Uptime</dt>
-      <dd class="font-medium text-gray-900">{{ h.uptimeSeconds }} s</dd>
+      <dd class="font-medium text-gray-900 dark:text-gray-100">{{ h.uptimeSeconds }} s</dd>
-      <dt class="text-gray-500">Status</dt>
+      <dt class="text-gray-500 dark:text-gray-400">Status</dt>
-      <dd class="font-medium text-green-700">{{ h.status }}</dd>
+      <dd class="font-medium text-green-700 dark:text-green-400">{{ h.status }}</dd>
     </dl>
     } @else if (isError()) {
-    <p class="mt-2 text-sm text-amber-700" role="status">
+    <p class="mt-2 text-sm text-amber-700 dark:text-amber-300" role="status">
       Backend unreachable. The BFF dev server may not be running, or CORS is misconfigured.
     </p>
     }

apps/portal-shell/src/app/state/layout-state.service.spec.ts

@@ -4,9 +4,11 @@ import { LayoutStateService } from './layout-state.service';
 describe('LayoutStateService', () => {
   beforeEach(() => {
     localStorage.clear();
+    document.documentElement.classList.remove('dark');
     TestBed.resetTestingModule();
   });
 
+  describe('sidebar state', () => {
     it('defaults to expanded when no preference is persisted', () => {
       const service = TestBed.inject(LayoutStateService);
       expect(service.sidebarCollapsed()).toBe(false);
@@ -40,4 +42,53 @@ describe('LayoutStateService', () => {
       TestBed.tick();
       expect(localStorage.getItem('portal-shell:sidebar-collapsed')).toBe('1');
     });
+  });
+
+  describe('theme state', () => {
+    it('defaults to auto when no preference is persisted', () => {
+      const service = TestBed.inject(LayoutStateService);
+      expect(service.themeMode()).toBe('auto');
+    });
+
+    it('initialises from the persisted theme mode', () => {
+      localStorage.setItem('portal-shell:theme-mode', 'dark');
+      const service = TestBed.inject(LayoutStateService);
+      expect(service.themeMode()).toBe('dark');
+    });
+
+    it('ignores an invalid persisted value and falls back to auto', () => {
+      localStorage.setItem('portal-shell:theme-mode', 'sepia' as unknown as string);
+      const service = TestBed.inject(LayoutStateService);
+      expect(service.themeMode()).toBe('auto');
+    });
+
+    it('setThemeMode persists the choice and toggles the .dark class on <html>', () => {
+      const service = TestBed.inject(LayoutStateService);
+      const html = document.documentElement;
+
+      service.setThemeMode('dark');
+      TestBed.tick();
+      expect(localStorage.getItem('portal-shell:theme-mode')).toBe('dark');
+      expect(html.classList.contains('dark')).toBe(true);
+      expect(service.effectiveTheme()).toBe('dark');
+
+      service.setThemeMode('light');
+      TestBed.tick();
+      expect(localStorage.getItem('portal-shell:theme-mode')).toBe('light');
+      expect(html.classList.contains('dark')).toBe(false);
+      expect(service.effectiveTheme()).toBe('light');
+    });
+
+    it('resolves auto via the system preference (jsdom defaults to light)', () => {
+      const service = TestBed.inject(LayoutStateService);
+      service.setThemeMode('auto');
+      TestBed.tick();
+      expect(service.themeMode()).toBe('auto');
+      // jsdom's matchMedia reports no `prefers-color-scheme: dark` match,
+      // so 'auto' resolves to light. We assert the resolution path —
+      // not the literal value of the system preference.
+      expect(service.effectiveTheme()).toBe('light');
+      expect(document.documentElement.classList.contains('dark')).toBe(false);
+    });
+  });
 });

apps/portal-shell/src/app/state/layout-state.service.ts

@@ -1,38 +1,84 @@
-import { Injectable, effect, signal } from '@angular/core';
+import { DOCUMENT } from '@angular/common';
+import { Injectable, computed, effect, inject, signal } from '@angular/core';
 
 /**
  * Shell-level layout state shared by the header and the sidebar.
  *
- * Currently a single piece of state: whether the sidebar is collapsed
+ * v1 surfaces two settings:
- * to its icon-only rail. The collapsed state is persisted in
+ *   - `sidebarCollapsed`: whether the sidebar is collapsed to its
- * `localStorage` so it survives reloads without a backend round-trip.
+ *     icon-only rail. The header reads it to size its logo zone in
+ *     lock-step with the sidebar.
+ *   - `themeMode`: the user's color-scheme preference among
+ *     `light` / `dark` / `auto`. `effectiveTheme` resolves it to a
+ *     concrete `'light' | 'dark'` by consulting the system preference
+ *     when in `auto` mode, and reacts to OS-level theme changes.
+ *     A side-effect toggles the `.dark` class on <html> so every
+ *     `dark:` Tailwind utility flips at once.
  *
- * Lives outside of `<app-sidebar>` so that `<app-header>` can also
+ * Both pieces of state persist in `localStorage` so the user's
- * read it — that's what lets the header's logo zone size itself to
+ * preferences survive a reload without a backend round-trip.
- * match the sidebar width, and stay perfectly aligned with the rail
+ *
- * below it. As more cross-cutting shell state lands (density, theme,
+ * Lives outside of the components that consume it so multiple
- * panel pinning, …), it joins here rather than spreading across
+ * surfaces stay synchronised without prop-drilling, and so the
- * sibling components.
+ * service is the natural home for future cross-cutting shell state
+ * (density, RTL, panel pinning, ...).
  */
 
-const STORAGE_KEY = 'portal-shell:sidebar-collapsed';
+const SIDEBAR_STORAGE_KEY = 'portal-shell:sidebar-collapsed';
+const THEME_STORAGE_KEY = 'portal-shell:theme-mode';
+const DARK_CLASS = 'dark';
+
+export type ThemeMode = 'light' | 'dark' | 'auto';
 
 @Injectable({ providedIn: 'root' })
 export class LayoutStateService {
-  private readonly _sidebarCollapsed = signal(readPersistedCollapsed());
+  private readonly document = inject(DOCUMENT);
+
+  private readonly _sidebarCollapsed = signal(readPersistedSidebarCollapsed());
+  private readonly _themeMode = signal<ThemeMode>(readPersistedThemeMode());
+  private readonly _systemPrefersDark = signal(systemPrefersDark(this.document));
 
   readonly sidebarCollapsed = this._sidebarCollapsed.asReadonly();
+  readonly themeMode = this._themeMode.asReadonly();
+  readonly effectiveTheme = computed<'light' | 'dark'>(() => {
+    const mode = this._themeMode();
+    if (mode === 'auto') {
+      return this._systemPrefersDark() ? 'dark' : 'light';
+    }
+    return mode;
+  });
 
   constructor() {
     effect(() => {
       const value = this._sidebarCollapsed();
       try {
-        localStorage.setItem(STORAGE_KEY, value ? '1' : '0');
+        localStorage.setItem(SIDEBAR_STORAGE_KEY, value ? '1' : '0');
       } catch {
         // localStorage can throw in private-mode contexts; the UI
         // still works, the preference just doesn't persist.
       }
     });
+
+    effect(() => {
+      const mode = this._themeMode();
+      try {
+        localStorage.setItem(THEME_STORAGE_KEY, mode);
+      } catch {
+        // See above.
+      }
+    });
+
+    effect(() => {
+      const theme = this.effectiveTheme();
+      const root = this.document.documentElement;
+      if (theme === 'dark') {
+        root.classList.add(DARK_CLASS);
+      } else {
+        root.classList.remove(DARK_CLASS);
+      }
+    });
+
+    this.watchSystemPreference();
   }
 
   toggleSidebar(): void {
@@ -42,12 +88,47 @@ export class LayoutStateService {
   setSidebarCollapsed(value: boolean): void {
     this._sidebarCollapsed.set(value);
   }
+
+  setThemeMode(mode: ThemeMode): void {
+    this._themeMode.set(mode);
+  }
+
+  private watchSystemPreference(): void {
+    const mql = this.document.defaultView?.matchMedia?.('(prefers-color-scheme: dark)');
+    if (!mql) {
+      return;
+    }
+    const listener = (e: MediaQueryListEvent) => this._systemPrefersDark.set(e.matches);
+    // `addEventListener` is the modern API; older Safari only exposes
+    // the deprecated `addListener` — guarded for robustness.
+    if (typeof mql.addEventListener === 'function') {
+      mql.addEventListener('change', listener);
+    } else {
+      mql.addListener(listener);
+    }
+  }
 }
 
-function readPersistedCollapsed(): boolean {
+function readPersistedSidebarCollapsed(): boolean {
   try {
-    return localStorage.getItem(STORAGE_KEY) === '1';
+    return localStorage.getItem(SIDEBAR_STORAGE_KEY) === '1';
   } catch {
     return false;
   }
 }
+
+function readPersistedThemeMode(): ThemeMode {
+  try {
+    const raw = localStorage.getItem(THEME_STORAGE_KEY);
+    if (raw === 'light' || raw === 'dark' || raw === 'auto') {
+      return raw;
+    }
+  } catch {
+    // fall through
+  }
+  return 'auto';
+}
+
+function systemPrefersDark(doc: Document): boolean {
+  return doc.defaultView?.matchMedia?.('(prefers-color-scheme: dark)').matches ?? false;
+}

apps/portal-shell/src/styles.css

@@ -9,6 +9,16 @@
 
 @import 'tailwindcss';
 
+/*
+ * Class-based dark mode. Tailwind v4 default is media-query-driven
+ * (`prefers-color-scheme`); we override it so a `.dark` class on
+ * <html> (set by `LayoutStateService` from the user's explicit
+ * preference, or computed from the system preference in `auto` mode)
+ * is the single switch. `&:where(.dark, .dark *)` keeps specificity
+ * neutral so utility ordering doesn't shift.
+ */
+@custom-variant dark (&:where(.dark, .dark *));
+
 /*
  * Brand palette — APF charte graphique.
  *

package.json

@@ -111,6 +111,7 @@
     "webpack-cli": "^5.1.4"
   },
   "dependencies": {
+    "@angular/cdk": "~21.2.10",
     "@angular/common": "~21.2.0",
     "@angular/compiler": "~21.2.0",
     "@angular/core": "~21.2.0",

pnpm-lock.yaml

@@ -17,6 +17,9 @@ importers:
 
   .:
     dependencies:
+      '@angular/cdk':
+        specifier: ~21.2.10
+        version: 21.2.10(@angular/common@21.2.12(@angular/core@21.2.12(@angular/compiler@21.2.12)(rxjs@7.8.2)(zone.js@0.16.2))(rxjs@7.8.2))(@angular/core@21.2.12(@angular/compiler@21.2.12)(rxjs@7.8.2)(zone.js@0.16.2))(@angular/platform-browser@21.2.12(@angular/common@21.2.12(@angular/core@21.2.12(@angular/compiler@21.2.12)(rxjs@7.8.2)(zone.js@0.16.2))(rxjs@7.8.2))(@angular/core@21.2.12(@angular/compiler@21.2.12)(rxjs@7.8.2)(zone.js@0.16.2)))(rxjs@7.8.2)
       '@angular/common':
         specifier: ~21.2.0
         version: 21.2.12(@angular/core@21.2.12(@angular/compiler@21.2.12)(rxjs@7.8.2)(zone.js@0.16.2))(rxjs@7.8.2)
@@ -575,6 +578,14 @@ packages:
       vitest:
         optional: true
 
+  '@angular/cdk@21.2.10':
+    resolution: {integrity: sha512-yfCzUFNfeSMNnCkc0P5Pozqz1EViDe9KLPQyHVY/hApsNgBWvckpA+ZEWgKNfAf72f8bUJvZoHejaSMGYrpvuw==}
+    peerDependencies:
+      '@angular/common': ^21.0.0 || ^22.0.0
+      '@angular/core': ^21.0.0 || ^22.0.0
+      '@angular/platform-browser': ^21.0.0 || ^22.0.0
+      rxjs: ^6.5.3 || ^7.4.0
+
   '@angular/cli@21.2.10':
     resolution: {integrity: sha512-ezf9LM0GgexG2l8ae/uN4fUyxGqeFEH9iu30mUMU5dwow76aK+b4Abuf5eSuR0F8zTLEA3ZUEYywI+gajbAUuA==}
     engines: {node: ^20.19.0 || ^22.12.0 || >=24.0.0, npm: ^6.11.0 || ^7.5.6 || >=8.0.0, yarn: '>= 1.13.0'}
@@ -10383,6 +10394,15 @@ snapshots:
       - tsx
       - yaml
 
+  '@angular/cdk@21.2.10(@angular/common@21.2.12(@angular/core@21.2.12(@angular/compiler@21.2.12)(rxjs@7.8.2)(zone.js@0.16.2))(rxjs@7.8.2))(@angular/core@21.2.12(@angular/compiler@21.2.12)(rxjs@7.8.2)(zone.js@0.16.2))(@angular/platform-browser@21.2.12(@angular/common@21.2.12(@angular/core@21.2.12(@angular/compiler@21.2.12)(rxjs@7.8.2)(zone.js@0.16.2))(rxjs@7.8.2))(@angular/core@21.2.12(@angular/compiler@21.2.12)(rxjs@7.8.2)(zone.js@0.16.2)))(rxjs@7.8.2)':
+    dependencies:
+      '@angular/common': 21.2.12(@angular/core@21.2.12(@angular/compiler@21.2.12)(rxjs@7.8.2)(zone.js@0.16.2))(rxjs@7.8.2)
+      '@angular/core': 21.2.12(@angular/compiler@21.2.12)(rxjs@7.8.2)(zone.js@0.16.2)
+      '@angular/platform-browser': 21.2.12(@angular/common@21.2.12(@angular/core@21.2.12(@angular/compiler@21.2.12)(rxjs@7.8.2)(zone.js@0.16.2))(rxjs@7.8.2))(@angular/core@21.2.12(@angular/compiler@21.2.12)(rxjs@7.8.2)(zone.js@0.16.2))
+      parse5: 8.0.1
+      rxjs: 7.8.2
+      tslib: 2.8.1
+
   '@angular/cli@21.2.10(@types/node@24.12.3)(chokidar@5.0.0)':
     dependencies:
       '@angular-devkit/architect': 0.2102.10(chokidar@5.0.0)