diff --git a/package.json b/package.json
index 8c4f06b6..aeb057d0 100644
--- a/package.json
+++ b/package.json
@@ -74,6 +74,7 @@
     "tough-cookie": "^5.1.2"
   },
   "relativeDependencies": {
+    "@data-fair/lib-vue": "../lib/packages/vue",
     "@data-fair/lib-vuetify": "../lib/packages/vuetify"
   }
 }
diff --git a/portal/app/components/layout/layout-nav-bar.vue b/portal/app/components/layout/layout-nav-bar.vue
index e080ee3a..dcf1e379 100644
--- a/portal/app/components/layout/layout-nav-bar.vue
+++ b/portal/app/components/layout/layout-nav-bar.vue
@@ -36,6 +36,7 @@
 
         <v-toolbar-items v-if="portalConfig.authentication !== 'none'">
           <notification-queue />
+          <theme-switcher :theme="portalConfig.theme" />
           <layout-personal-menu
             :login-color="navBarConfig.loginColor"
             :nav-bar-color="navBarConfig.color"
@@ -50,6 +51,7 @@
 import type { NavBar } from '#api/types/portal-config'
 import { useDisplay } from 'vuetify'
 import { useElementSize } from '@vueuse/core'
+import themeSwitcher from '@data-fair/lib-vuetify/theme-switcher.vue'
 
 const { navBarConfig } = defineProps<{
   navBarConfig: NavBar
diff --git a/portal/app/components/layout/layout-personal-app-bar.vue b/portal/app/components/layout/layout-personal-app-bar.vue
index 511b9164..28d84cf2 100644
--- a/portal/app/components/layout/layout-personal-app-bar.vue
+++ b/portal/app/components/layout/layout-personal-app-bar.vue
@@ -18,6 +18,7 @@
     <div id="agent-chat-appbar" class="d-flex align-center" />
     <v-toolbar-items>
       <notification-queue />
+      <theme-switcher :theme="portalConfig.theme" />
 
       <!-- Personal Menu -->
       <layout-personal-menu personal />
@@ -28,8 +29,9 @@
 <script setup lang="ts">
 import { VToolbar, VAppBar } from 'vuetify/components'
 import { mdiMenu, mdiMenuOpen } from '@mdi/js'
+import themeSwitcher from '@data-fair/lib-vuetify/theme-switcher.vue'
 
-const { preview } = usePortalStore()
+const { preview, portalConfig } = usePortalStore()
 const { personalDrawer, breadcrumbs } = useNavigationStore()
 const { t } = useI18n()
 
diff --git a/portal/app/plugins/03-vuetify.ts b/portal/app/plugins/03-vuetify.ts
index b26f9b9f..310afc6f 100644
--- a/portal/app/plugins/03-vuetify.ts
+++ b/portal/app/plugins/03-vuetify.ts
@@ -1,44 +1,51 @@
+import type { AppliedTheme, Theme } from '@data-fair/lib-vue/session.js'
 import { createUiNotif } from '@data-fair/lib-vue/ui-notif.js'
 import { fr, en } from 'vuetify/locale'
 
+// vuetify-nuxt-module's useState('vuetify:nuxt:ssr-client-hints') shape — we only read prefers-color-scheme
+interface VuetifySSRClientHints {
+  prefersColorScheme?: 'dark' | 'light' | 'no-preference'
+}
+
 export default defineNuxtPlugin((nuxtApp) => {
   const cspNonce = useNonce()
-  const themeCookie = useCookie<'default' | 'hc' | 'dark' | 'hc-dark'>('theme', { default: () => 'default' })
+  const themeCookie = useCookie<Theme>('theme', { default: () => 'system' })
   const langCookie = useCookie<'fr' | 'en'>('i18n_lang', { default: () => 'fr' })
 
   const portalConfig = useNuxtApp().$portal.config
-  let colors = portalConfig.theme.colors
-  let dark = false
-
-  if (themeCookie.value === 'hc' && portalConfig.theme.hcColors) {
-    colors = portalConfig.theme.hcColors
-  }
-  if (themeCookie.value === 'dark' && portalConfig.theme.darkColors) {
-    colors = portalConfig.theme.darkColors
-    dark = true
-  }
-  if (themeCookie.value === 'hc-dark' && portalConfig.theme.hcDarkColors) {
-    colors = portalConfig.theme.hcDarkColors
-    dark = true
-  }
 
   // https://nuxt.vuetifyjs.com/guide/advanced/runtime-hooks.html
   nuxtApp.hook('vuetify:before-create', ({ vuetifyOptions }) => {
+    const applied = resolvePortalTheme(themeCookie.value)
+    let colors = portalConfig.theme.colors
+    let dark = false
+    if (applied === 'hc' && portalConfig.theme.hcColors) {
+      colors = portalConfig.theme.hcColors
+    }
+    if (applied === 'dark' && portalConfig.theme.darkColors) {
+      colors = portalConfig.theme.darkColors
+      dark = true
+    }
+    if (applied === 'hc-dark' && portalConfig.theme.hcDarkColors) {
+      colors = portalConfig.theme.hcDarkColors
+      dark = true
+    }
+
     vuetifyOptions.locale = {
       locale: langCookie.value,
       fallback: 'en',
       messages: { fr, en }
     }
+
     vuetifyOptions.theme = {
       cspNonce,
-      defaultTheme: themeCookie.value,
+      defaultTheme: applied,
       themes: {
-        [themeCookie.value]: {
+        [applied]: {
           dark,
           colors,
           variables: {
-            // deactivate automatic partial transparencies
-            // best to control colors precisely and ensure sufficient contrast for readability
+            // disable partial transparencies for precise color control and readable contrast
             'high-emphasis-opacity': 1,
             'medium-emphasis-opacity': 0.87
           }
@@ -50,4 +57,27 @@ export default defineNuxtPlugin((nuxtApp) => {
   nuxtApp.hook('vuetify:ready', () => {
     nuxtApp.vueApp.use(createUiNotif())
   })
+
+  // SSR variant of resolveTheme: on 'system', read prefers-color-scheme from the SSR Client Hint
+  // (exposed by vuetify-nuxt-module's ssrClientHints) and fall back to matchMedia on the client.
+  // Sec-CH-Forced-Colors exists but vuetify-nuxt-module doesn't expose it, so HC can't be
+  // auto-picked at SSR — it requires an explicit user choice.
+  function resolvePortalTheme (theme: Theme): AppliedTheme {
+    if (theme !== 'system') return theme
+
+    let prefersDark = false
+    let prefersHC = false
+    if (import.meta.server) {
+      const hints = useState<VuetifySSRClientHints | undefined>('vuetify:nuxt:ssr-client-hints')
+      prefersDark = hints.value?.prefersColorScheme === 'dark'
+    } else {
+      prefersDark = window.matchMedia?.('(prefers-color-scheme: dark)').matches ?? false
+      prefersHC = window.matchMedia?.('(forced-colors: active)').matches ?? false
+    }
+
+    if (portalConfig.theme.hcDark && prefersDark && prefersHC) return 'hc-dark'
+    if (portalConfig.theme.hc && prefersHC) return 'hc'
+    if (portalConfig.theme.dark && prefersDark) return 'dark'
+    return 'default'
+  }
 })
diff --git a/portal/nuxt.config.ts b/portal/nuxt.config.ts
index 140715e9..9b4e4603 100644
--- a/portal/nuxt.config.ts
+++ b/portal/nuxt.config.ts
@@ -123,7 +123,8 @@ export default defineNuxtConfig({
       },
       ssrClientHints: {
         // reloadOnFirstRequest: false, // disabled because broken with Brave unfortunately
-        viewportSize: true
+        viewportSize: true, // help SSR choose the right breakpoint on first load to avoid hydration mismatches
+        prefersColorScheme: true // handled manually in 03-vuetify.ts to avoid conflicts with our 'system' cookie value semantics
       }
     },
     vuetifyOptions: {