diff --git a/packages/types-builder/build.ts b/packages/types-builder/build.ts
index ac17fea..df2f7d0 100755
--- a/packages/types-builder/build.ts
+++ b/packages/types-builder/build.ts
@@ -359,6 +359,7 @@ const emit = defineEmits(emits)
         for (const locale of vjsfLocales) {
           const schemaVjsfOpts = { ...schema['x-vjsf'] }
           delete schemaVjsfOpts.compName
+          console.log(`  compiledLayout ${locale} options: ${JSON.stringify(schemaVjsfOpts)}`)
           const otherSchemas = { ...schemas }
           for (const [key, otherSchema] of Object.entries(schemas)) {
             if (key === schema.$id) continue
@@ -375,7 +376,6 @@ const emit = defineEmits(emits)
             fullOptions.components[componentInfo.name] = componentInfo
           }
 
-          console.log(`  compiledLayout ${locale} options: ${JSON.stringify(schemaVjsfOpts)}`)
           const compiledLayout = compileLayout(schema, fullOptions)
           let compiledLayoutCode = await serializeCompiledLayout(compiledLayout)
           // The serialized code declares `const compiledLayout = {...}`.
diff --git a/packages/vue/session.ts b/packages/vue/session.ts
index dbe62db..41fd02c 100644
--- a/packages/vue/session.ts
+++ b/packages/vue/session.ts
@@ -82,7 +82,8 @@ export interface SiteInfo {
   owner: AccountKeys
 }
 
-type Theme = 'default' | 'dark' | 'hc' | 'hc-dark'
+type AppliedTheme = 'default' | 'dark' | 'hc' | 'hc-dark'
+type Theme = AppliedTheme | 'system'
 
 export interface Session {
   state: SessionState
@@ -120,16 +121,21 @@ export type SessionAuthenticated = Omit<Session, 'state' | 'user' | 'account' |
 const debug = Debug('session')
 debug.log = console.log.bind(console)
 
-function getDefaultTheme (site: FullSiteInfo): Theme {
+function getDefaultTheme (site: FullSiteInfo): AppliedTheme {
   // see https://www.scottohara.me/blog/2021/10/01/detect-high-contrast-and-dark-modes.html
-  const preferDark = window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)').matches
-  const preferHC = window.matchMedia && window.matchMedia('(forced-colors: active)').matches
+  const preferDark = typeof window !== 'undefined' && window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)').matches
+  const preferHC = typeof window !== 'undefined' && window.matchMedia && window.matchMedia('(forced-colors: active)').matches
   if (site.theme.hcDark && preferDark && preferHC) return 'hc-dark'
   if (site.theme.hc && preferHC) return 'hc'
   if (site.theme.dark && preferDark) return 'dark'
   return 'default'
 }
 
+function resolveTheme (userTheme: Theme | null, site: FullSiteInfo): AppliedTheme {
+  if (userTheme && userTheme !== 'system') return userTheme
+  return getDefaultTheme(site)
+}
+
 function jwtDecodeAlive (jwt: string | null): User | undefined {
   if (!jwt) return
   const decoded = jwtDecode(jwt) as any
@@ -403,6 +409,9 @@ export async function getSession (initOptions: Partial<SessionOptions>): Promise
   const setSiteInfo = (siteInfo: any) => {
     if (siteInfo.theme) {
       fullSite.value = siteInfo
+      // an absent cookie is treated as an implicit 'system' choice so the
+      // theme-switcher radio has a value to bind to.
+      if (theme.value == null) theme.value = 'system'
       const partialSite: SiteInfo = {
         main: siteInfo.main,
         isAccountMain: siteInfo.isAccountMain,
@@ -412,13 +421,13 @@ export async function getSession (initOptions: Partial<SessionOptions>): Promise
         authOnlyOtherSite: siteInfo.authOnlyOtherSite,
         owner: siteInfo.owner
       }
-      if (theme.value == null) theme.value = getDefaultTheme(siteInfo)
-      if (theme.value === 'hc') partialSite.colors = siteInfo.theme.hcColors
-      if (theme.value === 'dark') {
+      const applied = resolveTheme(theme.value, siteInfo)
+      if (applied === 'hc') partialSite.colors = siteInfo.theme.hcColors
+      if (applied === 'dark') {
         partialSite.colors = siteInfo.theme.darkColors
         partialSite.dark = true
       }
-      if (theme.value === 'hc-dark') {
+      if (applied === 'hc-dark') {
         partialSite.colors = siteInfo.theme.hcDarkColors
         partialSite.dark = true
       }
@@ -433,6 +442,16 @@ export async function getSession (initOptions: Partial<SessionOptions>): Promise
   // @ts-ignore
   if (!ssr && window.__PUBLIC_SITE_INFO) setSiteInfo(window.__PUBLIC_SITE_INFO)
 
+  // re-apply the theme when the OS preference changes while the user is on
+  // 'system'. Important for mobile devices that switch light/dark over the day.
+  if (!ssr && typeof window !== 'undefined' && window.matchMedia) {
+    const onOsPrefChange = () => {
+      if (theme.value === 'system' && fullSite.value) setSiteInfo(fullSite.value)
+    }
+    window.matchMedia('(prefers-color-scheme: dark)').addEventListener('change', onOsPrefChange)
+    window.matchMedia('(forced-colors: active)').addEventListener('change', onOsPrefChange)
+  }
+
   // immediately performs a keepalive, but only on top windows (not iframes or popups)
   // and only if it was not done very recently (maybe from a refreshed page next to this one)
   // also run an auto-refresh loop
diff --git a/packages/vuetify/layout-fetch-error.vue b/packages/vuetify/layout-fetch-error.vue
index f56e0bc..57d10fe 100644
--- a/packages/vuetify/layout-fetch-error.vue
+++ b/packages/vuetify/layout-fetch-error.vue
@@ -64,7 +64,7 @@ const { error, backTo = '/', backLabel } = defineProps<{
 }>()
 
 const { t } = useI18n({ useScope: 'local' })
-const { switchOrganization, user } = useSession()
+const { switchOrganization, user, account } = useSession()
 
 const statusCode = computed(() => error?.statusCode ?? error?.status ?? 500)
 
@@ -99,9 +99,25 @@ const switchOrg = computed(() => {
   try { owner = JSON.parse(rawOwner) } catch { return null }
   if (!owner || owner.type !== 'organization' || !owner.id) return null
 
-  return user.value.organizations?.find(o =>
-    o.id === owner!.id && (o.department ?? undefined) === (owner!.department ?? undefined)
-  ) ?? null
+  const orgs = user.value.organizations ?? []
+  const ownerDept = owner.department ?? undefined
+  const isCurrentAccount = (o: { id: string, department?: string }) =>
+    account.value?.type === 'organization' &&
+    account.value.id === o.id &&
+    (account.value.department ?? undefined) === (o.department ?? undefined)
+
+  // Prefer a membership matching the resource's department exactly...
+  const exact = orgs.find(o =>
+    o.id === owner!.id && (o.department ?? undefined) === ownerDept && !isCurrentAccount(o)
+  )
+  if (exact) return exact
+  // ...otherwise fall back to a membership at the organization root: in
+  // simple-directory's authz model, root access generally grants visibility
+  // over department-scoped resources.
+  if (ownerDept) {
+    return orgs.find(o => o.id === owner!.id && !o.department && !isCurrentAccount(o)) ?? null
+  }
+  return null
 })
 
 const doSwitch = () => {
diff --git a/packages/vuetify/personal-menu.vue b/packages/vuetify/personal-menu.vue
index 83e0410..508349c 100644
--- a/packages/vuetify/personal-menu.vue
+++ b/packages/vuetify/personal-menu.vue
@@ -182,9 +182,8 @@ fr:
   openPersonalMenu: Ouvrez le menu personnel
   personalAccount: Compte personnel
   switchAccount: Changer de compte
-  adminMode: mode admin
+  adminMode: Mode admin
   backToAdmin: Revenir à ma session administrateur
-  darkMode: mode nuit
   plannedDeletion: La suppression de l'utilisateur {name} et toutes ses informations est programmée le {plannedDeletion}.
   cancelDeletion: Annuler la suppression de l'utilisateur
 en:
@@ -193,9 +192,8 @@ en:
   openPersonalMenu: Open personal menu
   personalAccount: Personal account
   switchAccount: Switch account
-  adminMode: admin mode
+  adminMode: Admin mode
   backToAdmin: Return to administrator session
-  darkMode: night mode
   plannedDeletion: The deletion of the user {name} and all its data is planned on the {plannedDeletion}.
   cancelDeletion: Cancel the deletion of the user
 </i18n>
diff --git a/packages/vuetify/theme-switcher.vue b/packages/vuetify/theme-switcher.vue
index 16f18cd..b57abb3 100644
--- a/packages/vuetify/theme-switcher.vue
+++ b/packages/vuetify/theme-switcher.vue
@@ -22,8 +22,9 @@
             color="primary"
             hide-details
             :label="t('themeSwitch')"
-            @update:modelValue="value => session.switchTheme(value as 'default' | 'dark' | 'hc' | 'hc-dark')"
+            @update:modelValue="value => session.switchTheme(value as 'default' | 'dark' | 'hc' | 'hc-dark' | 'system')"
           >
+            <v-radio :label="t('theme.system')" value="system"></v-radio>
             <v-radio :label="t('theme.default')" value="default"></v-radio>
             <v-radio v-if="session.fullSite.value?.theme.dark" :label="t('theme.dark')" value="dark"></v-radio>
             <v-radio v-if="session.fullSite.value?.theme.hc":label="t('theme.hc')" value="hc"></v-radio>
@@ -39,17 +40,19 @@
 fr:
   themeSwitch: Changer de thème
   theme:
-    default: par défaut
-    dark: sombre
-    hc: contraste élevé
-    hcDark: sombre et contraste élevé
+    system: Système
+    default: Par défaut
+    dark: Sombre
+    hc: Contraste élevé
+    hcDark: Sombre et contraste élevé
 en:
   themeSwitch: Change theme
   theme:
-    default: default
-    dark: dark
-    hc: high contrast
-    hcDark: dark and high contrast
+    system: System
+    default: Default
+    dark: Dark
+    hc: High contrast
+    hcDark: Dark and high contrast
 </i18n>
 
 <script setup lang="ts">