diff --git a/core/l10n/en.json b/core/l10n/en.json
index 8158294..b2e8659 100644
--- a/core/l10n/en.json
+++ b/core/l10n/en.json
@@ -9,6 +9,7 @@
     "darkMode": "Dark Mode",
     "lightMode": "Light Mode",
     "logout": "Logout",
-    "settings": "Settings"
+    "settings": "Settings",
+    "systemMode": "System Mode"
   }
 }
diff --git a/core/l10n/en.manual.json b/core/l10n/en.manual.json
new file mode 100644
index 0000000..90bc62e
--- /dev/null
+++ b/core/l10n/en.manual.json
@@ -0,0 +1,7 @@
+{
+  "userMenu": {
+    "darkMode": "Dark Mode",
+    "lightMode": "Light Mode",
+    "systemMode": "System Mode"
+  }
+}
diff --git a/core/lib/Controllers/TenantSettingsController.php b/core/lib/Controllers/TenantSettingsController.php
index ebb66fc..dd8101b 100644
--- a/core/lib/Controllers/TenantSettingsController.php
+++ b/core/lib/Controllers/TenantSettingsController.php
@@ -49,9 +49,9 @@ class TenantSettingsController extends ControllerAbstract
      * @example request body:
      * {
      *   "data": {
-     *     "default_mode": "dark",
-     *     "primary_color": "#6366F1",
-     *     "lock_user_colors": true
+     *     "theme_default_mode": "dark",
+     *     "theme_palette": {"light": {"colors": {"primary": "#0284C7"}}},
+     *     "theme_lock": true
      *   }
      * }
      *
diff --git a/core/src/App.vue b/core/src/App.vue
index 90f7a91..b06a0d7 100644
--- a/core/src/App.vue
+++ b/core/src/App.vue
@@ -5,69 +5,5 @@
 
 <script setup lang="ts">
 import { RouterView } from 'vue-router';
-import { onMounted, watch } from 'vue';
-import { useTheme } from 'vuetify';
 import SharedSnackbar from '@KTXC/components/shared/SharedSnackbar.vue';
-import { useLayoutStore } from '@KTXC/stores/layoutStore';
-import { useUserStore } from '@KTXC/stores/userStore';
-import { useTenantStore } from '@KTXC/stores/tenantStore';
-
-const theme = useTheme();
-const layoutStore = useLayoutStore();
-const userStore = useUserStore();
-const tenantStore = useTenantStore();
-
-// Maps user/tenant setting keys → Vuetify color token names
-const COLOR_SETTINGS: Array<{ token: string; key: string }> = [
-  { token: 'primary',   key: 'primary_color' },
-  { token: 'secondary', key: 'secondary_color' },
-];
-
-/**
- * Apply brand color overrides from stored preferences to all Vuetify theme
- * variants (light & dark).  Tenant colors take priority when lock is active.
- */
-function applyThemeColors(): void {
-  const locked = tenantStore.getSetting('lock_user_colors') as boolean | null;
-
-  for (const { token, key } of COLOR_SETTINGS) {
-    const value = locked
-      ? ((tenantStore.getSetting(key) as string | null) ?? (userStore.getSetting(key) as string | null))
-      : ((userStore.getSetting(key) as string | null) ?? (tenantStore.getSetting(key) as string | null));
-
-    if (value) {
-      for (const variant of Object.keys(theme.themes.value)) {
-        theme.themes.value[variant].colors[token] = value;
-      }
-    }
-  }
-}
-
-/** Apply font preference via CSS custom property and body style. */
-function applyFont(): void {
-  const font =
-    ((userStore.getSetting('font') as string | null) ??
-     (tenantStore.getSetting('font') as string | null));
-
-  if (font && font !== 'Public Sans') {
-    document.documentElement.style.setProperty('--themer-font', font);
-    document.body.style.fontFamily = `"${font}", sans-serif`;
-  }
-}
-
-onMounted(() => {
-  // Apply saved theme mode
-  if (layoutStore.theme) {
-    theme.global.name.value = layoutStore.theme;
-  }
-
-  applyThemeColors();
-  applyFont();
-});
-
-// Re-apply whenever tenant settings change (e.g. admin saves new brand colors)
-watch(() => tenantStore.settings, () => {
-  applyThemeColors();
-  applyFont();
-}, { deep: true });
 </script>
diff --git a/core/src/layouts/menus/LayoutUserMenu.vue b/core/src/layouts/menus/LayoutUserMenu.vue
index 300b966..67ddf4e 100644
--- a/core/src/layouts/menus/LayoutUserMenu.vue
+++ b/core/src/layouts/menus/LayoutUserMenu.vue
@@ -1,6 +1,5 @@
 <script setup lang="ts">
 import { computed } from 'vue';
-import { useTheme } from 'vuetify';
 import { useUserStore } from '@KTXC/stores/userStore';
 import { useIntegrationStore } from '@KTXC/stores/integrationStore';
 import { useLayoutStore } from '@KTXC/stores/layoutStore';
@@ -10,7 +9,6 @@ import defaultAvatar from '@KTXC/assets/images/users/avatar-1.png';
 
 const { t } = useL10n('core');
 
-const theme = useTheme();
 const router = useRouter();
 const userStore = useUserStore();
 const integrationStore = useIntegrationStore();
@@ -27,12 +25,22 @@ const userName = computed(() => {
 });
 const userEmail = computed(() => userStore.getProfileField('email') || '');
 
-// Theme toggle
-const isDarkMode = computed(() => theme.global.name.value === 'dark');
-const toggleTheme = () => {
-  const newTheme = theme.global.name.value === 'light' ? 'dark' : 'light';
-  theme.global.name.value = newTheme;
-  layoutStore.setTheme(newTheme);
+// Theme mode cycle: light -> dark -> system, driven by the stored preference
+// (not the resolved theme name, which never reads 'system')
+const THEME_MODES = ['light', 'dark', 'system'] as const;
+type ThemeMode = (typeof THEME_MODES)[number];
+
+const MODE_PRESENTATION: Record<ThemeMode, { icon: string; l10n: string; label: string }> = {
+  light: { icon: 'mdi-weather-sunny', l10n: 'userMenu.lightMode', label: 'Light Mode' },
+  dark: { icon: 'mdi-weather-night', l10n: 'userMenu.darkMode', label: 'Dark Mode' },
+  system: { icon: 'mdi-theme-light-dark', l10n: 'userMenu.systemMode', label: 'System Mode' },
+};
+
+const themeMode = computed(() => (layoutStore.theme ?? 'light') as ThemeMode);
+const nextThemeMode = computed(() => THEME_MODES[(THEME_MODES.indexOf(themeMode.value) + 1) % THEME_MODES.length]);
+const cycleTheme = () => {
+  // App.vue watches layoutStore.theme and applies it via theme.change()
+  layoutStore.setTheme(nextThemeMode.value);
 };
 
 // Navigate to settings
@@ -82,12 +90,12 @@ const goToSettings = () => {
 
         <v-divider v-if="profileMenuItems.length" class="my-2" />
 
-        <!-- Theme Toggle -->
-        <v-list-item @click="toggleTheme" color="primary" rounded="0">
+        <!-- Theme Mode Cycle (light -> dark -> system) -->
+        <v-list-item @click="cycleTheme" color="primary" rounded="0">
           <template v-slot:prepend>
-            <v-icon>{{ isDarkMode ? 'mdi-weather-sunny' : 'mdi-weather-night' }}</v-icon>
+            <v-icon>{{ MODE_PRESENTATION[nextThemeMode].icon }}</v-icon>
           </template>
-          <v-list-item-title class="text-h6">{{ isDarkMode ? t('userMenu.lightMode', 'Light Mode') : t('userMenu.darkMode', 'Dark Mode') }}</v-list-item-title>
+          <v-list-item-title class="text-h6">{{ t(MODE_PRESENTATION[nextThemeMode].l10n, MODE_PRESENTATION[nextThemeMode].label) }}</v-list-item-title>
         </v-list-item>
 
         <!-- Go to Settings -->
diff --git a/core/src/private.ts b/core/src/private.ts
index 44c4a4b..096a160 100644
--- a/core/src/private.ts
+++ b/core/src/private.ts
@@ -8,6 +8,7 @@ import { useModuleStore } from '@KTXC/stores/moduleStore'
 import { useTenantStore } from '@KTXC/stores/tenantStore'
 import { useUserStore } from '@KTXC/stores/userStore'
 import { useL10nStore } from '@KTXC/stores/l10nStore'
+import { useThemeStore } from '@KTXC/stores/themeStore'
 import { i18n } from '@KTXC/l10n/runtime'
 import { fetchWrapper } from '@KTXC/utils/helpers/fetch-wrapper'
 import { FetchError } from '@KTXC/utils/helpers/fetch-wrapper-core'
@@ -41,12 +42,14 @@ globalWindow.Pinia = PiniaLib as unknown
 	const tenantStore = useTenantStore();
 	const userStore = useUserStore();
 	const l10nStore = useL10nStore();
+	const themeStore = useThemeStore();
 
 	try {
 		const payload = await fetchWrapper.get('/init');
 		moduleStore.init(payload?.modules ?? {});
 		tenantStore.init(payload?.tenant ?? null);
 		userStore.init(payload?.user ?? {});
+		themeStore.boot();
 
 		// Resolve locale and load catalogs before modules boot
 		await l10nStore.init(payload?.l10n ?? null);
diff --git a/core/src/shared/index.ts b/core/src/shared/index.ts
index 11888f7..0b7604e 100644
--- a/core/src/shared/index.ts
+++ b/core/src/shared/index.ts
@@ -13,6 +13,8 @@ export { useUserStore } from '../stores/userStore'
 export { useIntegrationStore } from '../stores/integrationStore'
 export { useLayoutStore } from '../stores/layoutStore'
 export { useL10nStore } from '../stores/l10nStore'
+export { useThemeStore, DEFAULT_FONT, encodePaletteSet } from '../stores/themeStore'
+export type { ThemePalette, ThemePaletteSet, ThemeVariant } from '../stores/themeStore'
 
 // Composables
 export { useL10n, t } from '../composables/useL10n'
diff --git a/core/src/stores/layoutStore.ts b/core/src/stores/layoutStore.ts
index b161680..cdf9fcd 100644
--- a/core/src/stores/layoutStore.ts
+++ b/core/src/stores/layoutStore.ts
@@ -1,8 +1,10 @@
 import { ref, watch } from 'vue';
 import { defineStore } from 'pinia';
 import { useUserStore } from './userStore';
+import { useTenantStore } from './tenantStore';
 
 export type MenuMode = 'apps' | 'user-settings' | 'admin-settings';
+export type ThemeMode = 'light' | 'dark' | 'system';
 
 export const useLayoutStore = defineStore('layout', () => {
   // Loading state
@@ -10,13 +12,17 @@ export const useLayoutStore = defineStore('layout', () => {
 
   // Sidebar state - initialize from settings or defaults
   const userStore = useUserStore();
+  const tenantStore = useTenantStore();
   const sidebarDrawer = ref(userStore.getSetting('sidebar_drawer') ?? true);
   const miniSidebar = ref(userStore.getSetting('mini_sidebar') ?? false);
   const menuMode = ref<MenuMode>('apps');
 
-  // Theme state - initialize from settings or defaults
-  const theme = ref(userStore.getSetting('theme') ?? 'light');
-  const font = ref(userStore.getSetting('font') ?? 'Public sans');
+  // Theme mode - user choice, falling back to the tenant default
+  const theme = ref<ThemeMode>(
+    (userStore.getSetting('theme') as ThemeMode | null) ??
+    (tenantStore.getSetting('theme_default_mode') as ThemeMode | null) ??
+    'light'
+  );
 
   // Watch and sync sidebar state to settings
   watch(sidebarDrawer, (value) => {
@@ -31,10 +37,6 @@ export const useLayoutStore = defineStore('layout', () => {
     userStore.setSetting('theme', value);
   });
 
-  watch(font, (value) => {
-    userStore.setSetting('font', value);
-  });
-
   // Actions
   function toggleSidebarDrawer() {
     sidebarDrawer.value = !sidebarDrawer.value;
@@ -44,14 +46,10 @@ export const useLayoutStore = defineStore('layout', () => {
     miniSidebar.value = value;
   }
 
-  function setTheme(value: string) {
+  function setTheme(value: ThemeMode) {
     theme.value = value;
   }
 
-  function setFont(value: string) {
-    font.value = value;
-  }
-
   function setMenuMode(value: MenuMode) {
     menuMode.value = value;
   }
@@ -71,14 +69,12 @@ export const useLayoutStore = defineStore('layout', () => {
     miniSidebar,
     menuMode,
     theme,
-    font,
 
     // Actions
     toggleSidebarDrawer,
     setMiniSidebar,
     setMenuMode,
     toggleMenuMode,
-    setTheme,
-    setFont
+    setTheme
   };
 });
diff --git a/core/src/stores/themeStore.ts b/core/src/stores/themeStore.ts
new file mode 100644
index 0000000..87d8db6
--- /dev/null
+++ b/core/src/stores/themeStore.ts
@@ -0,0 +1,194 @@
+import { computed, watch } from 'vue';
+import { defineStore } from 'pinia';
+import vuetify from '@KTXC/plugins/vuetify/index';
+import { useUserStore } from './userStore';
+import { useTenantStore } from './tenantStore';
+import { useLayoutStore } from './layoutStore';
+
+export const DEFAULT_FONT = 'Public Sans';
+
+export type ThemeVariant = 'light' | 'dark';
+
+/** Sparse palette overrides for one theme variant */
+export interface ThemePalette {
+  colors: Record<string, string>;
+  variables?: Record<string, string | number>;
+}
+
+/** The shape persisted in the `theme_palette` user/tenant setting */
+export type ThemePaletteSet = Partial<Record<ThemeVariant, ThemePalette>>;
+
+interface VariantSnapshot {
+  colors: Record<string, string>;
+  variables: Record<string, string | number>;
+}
+
+const VARIANTS: ThemeVariant[] = ['light', 'dark'];
+
+export function encodePaletteSet(paletteSet: ThemePaletteSet | null): string | null {
+  return paletteSet ? JSON.stringify(paletteSet) : null;
+}
+
+function parsePaletteSet(value: unknown): ThemePaletteSet | null {
+  if (!value) return null;
+  if (typeof value === 'string') {
+    try {
+      return JSON.parse(value) as ThemePaletteSet;
+    } catch {
+      return null;
+    }
+  }
+  return null;
+}
+
+/**
+ * System theming engine.
+ *
+ * Applies the appearance settings (mode, palette, font) persisted in the
+ * user/tenant settings to the live Vuetify instance. Booted from private.ts
+ * before app.mount(), so the first paint is already themed. Settings are
+ * plain data — UI for editing them is provided by the themer module, which
+ * only reads/writes settings and never touches Vuetify directly.
+ *
+ * Setting keys consumed:
+ * - user:   `theme` (light|dark|system), `theme_palette`, `theme_font`
+ * - tenant: `theme_default_mode`, `theme_palette`, `theme_font`,
+ *           `theme_lock` (forces the tenant palette/font for all users)
+ */
+export const useThemeStore = defineStore('theme', () => {
+  const userStore = useUserStore();
+  const tenantStore = useTenantStore();
+  const layoutStore = useLayoutStore();
+
+  const theme = vuetify.theme;
+
+  // Frozen copies of the built-in palettes, captured before the first
+  // apply(). Stored palettes are sparse and always merge over these, so no
+  // token (grey scale, scrollbar, variables, ...) can vanish.
+  const defaults: Record<ThemeVariant, VariantSnapshot> = {
+    light: snapshotVariant('light'),
+    dark: snapshotVariant('dark'),
+  };
+
+  function snapshotVariant(variant: ThemeVariant): VariantSnapshot {
+    const definition = theme.themes.value[variant];
+    return Object.freeze({
+      colors: Object.freeze({ ...(definition?.colors ?? {}) }) as Record<string, string>,
+      variables: Object.freeze({ ...(definition?.variables ?? {}) }) as Record<string, string | number>,
+    });
+  }
+
+  // ===========================================================================
+  // Effective settings (tenant lock > user choice > tenant default > built-in)
+  // ===========================================================================
+
+  const isLocked = computed(() => tenantStore.getSetting('theme_lock') === true);
+
+  const effectivePalette = computed<ThemePaletteSet>(() => {
+    const tenantPalette = parsePaletteSet(tenantStore.getSetting('theme_palette'));
+    if (isLocked.value) return tenantPalette ?? {};
+    const userPalette = parsePaletteSet(userStore.getSetting('theme_palette'));
+    return userPalette ?? tenantPalette ?? {};
+  });
+
+  const effectiveFont = computed<string>(() => {
+    const tenantFont = tenantStore.getSetting('theme_font') as string | null;
+    if (isLocked.value) return tenantFont ?? DEFAULT_FONT;
+    const userFont = userStore.getSetting('theme_font') as string | null;
+    return userFont ?? tenantFont ?? DEFAULT_FONT;
+  });
+
+  // ===========================================================================
+  // Application
+  // ===========================================================================
+
+  function mergeVariant(variant: ThemeVariant, palette: ThemePalette | undefined): VariantSnapshot {
+    const overrides = palette?.colors ?? {};
+    const colors: Record<string, string> = { ...defaults[variant].colors, ...overrides };
+    // Any token overridden without an explicit foreground pair loses the
+    // inherited `on-*` value, so Vuetify regenerates it with luma-based
+    // contrast (arbitrary user colors stay readable).
+    for (const token of Object.keys(overrides)) {
+      if (!token.startsWith('on-') && !(`on-${token}` in overrides)) {
+        delete colors[`on-${token}`];
+      }
+    }
+    const variables = { ...defaults[variant].variables, ...(palette?.variables ?? {}) };
+    return { colors, variables };
+  }
+
+  function applyPalette(): void {
+    const paletteSet = effectivePalette.value;
+    for (const variant of VARIANTS) {
+      const target = theme.themes.value[variant];
+      if (!target) continue;
+      const merged = mergeVariant(variant, paletteSet[variant]);
+      // Wholesale assignment on the themes ref is reactive in Vuetify 4.0.x:
+      // computedThemes/styles regenerate the theme stylesheet.
+      target.colors = merged.colors as typeof target.colors;
+      target.variables = merged.variables;
+    }
+  }
+
+  function applyFont(font: string): void {
+    if (font && font !== DEFAULT_FONT) {
+      document.documentElement.style.setProperty('--themer-font', font);
+      document.body.style.fontFamily = `"${font}", sans-serif`;
+    } else {
+      document.documentElement.style.removeProperty('--themer-font');
+      document.body.style.fontFamily = '';
+    }
+  }
+
+  function applyMode(): void {
+    // 'system' resolves via Vuetify's built-in prefers-color-scheme tracking
+    theme.change(layoutStore.theme);
+  }
+
+  let booted = false;
+
+  /** Called from private.ts after the settings stores init, before app.mount(). */
+  function boot(): void {
+    if (booted) return;
+    booted = true;
+    watch(effectivePalette, applyPalette);
+    watch(effectiveFont, applyFont);
+    watch(() => layoutStore.theme, applyMode);
+    applyPalette();
+    applyFont(effectiveFont.value);
+    applyMode();
+  }
+
+  // ===========================================================================
+  // Preview themes (used by editing UIs; never persisted)
+  // ===========================================================================
+
+  /** Register/update a throwaway named theme, merged over the built-in defaults. */
+  function setPreviewTheme(name: string, variant: ThemeVariant, palette: ThemePalette): void {
+    const merged = mergeVariant(variant, palette);
+    theme.themes.value[name] = {
+      dark: variant === 'dark',
+      colors: merged.colors,
+      variables: merged.variables,
+    } as (typeof theme.themes.value)[string];
+  }
+
+  function removePreviewTheme(name: string): void {
+    delete theme.themes.value[name];
+  }
+
+  return {
+    // State
+    defaults,
+    isLocked,
+    effectivePalette,
+    effectiveFont,
+
+    // Lifecycle
+    boot,
+
+    // Preview support
+    setPreviewTheme,
+    removePreviewTheme,
+  };
+});