diff --git a/public/locales/ar-AR/dashboard.json b/public/locales/ar-AR/dashboard.json
index 60056924..ddc72564 100644
--- a/public/locales/ar-AR/dashboard.json
+++ b/public/locales/ar-AR/dashboard.json
@@ -266,6 +266,8 @@
     "fileIdVar": "معرف الملف",
     "userIdVar": "معرف المستخدم، يكون فارغًا في حالة عدم تسجيل الدخول.",
     "userDisplayNameVar": "اسم عرض المستخدم المرمز كعنوان URL.",
+    "themeVar": "السمة الحالية، \"light\" أو \"dark\".",
+    "darkVar": "ما إذا كانت السمة الداكنة مفعّلة، \"1\" أو \"0\".",
     "fileViewers": "تطبيقات الملفات",
     "viewerList": "قائمة التطبيقات",
     "defaultViewerApps": "التطبيقات الافتراضية",
diff --git a/public/locales/de-DE/dashboard.json b/public/locales/de-DE/dashboard.json
index a99f5e61..ba162936 100644
--- a/public/locales/de-DE/dashboard.json
+++ b/public/locales/de-DE/dashboard.json
@@ -267,6 +267,8 @@
     "fileIdVar": "Datei-ID",
     "userIdVar": "Benutzer-ID, leer wenn nicht angemeldet.",
     "userDisplayNameVar": "URL-kodierter Benutzer-Anzeigename.",
+    "themeVar": "Aktuelles Design, \"light\" oder \"dark\".",
+    "darkVar": "Ob das dunkle Design aktiv ist, \"1\" oder \"0\".",
     "fileViewers": "Dateianwendungen",
     "viewerList": "Anwendungsliste",
     "defaultViewerApps": "Standardanwendungen",
diff --git a/public/locales/en-US/dashboard.json b/public/locales/en-US/dashboard.json
index 073473b7..b8028538 100644
--- a/public/locales/en-US/dashboard.json
+++ b/public/locales/en-US/dashboard.json
@@ -266,6 +266,8 @@
     "fileIdVar": "File ID",
     "userIdVar": "User ID, empty when not logged in.",
     "userDisplayNameVar": "URL-encoded user display name.",
+    "themeVar": "Current theme, \"light\" or \"dark\".",
+    "darkVar": "Whether dark theme is active, \"1\" or \"0\".",
     "fileViewers": "File applications",
     "viewerList": "Application list",
     "defaultViewerApps": "Default applications",
diff --git a/public/locales/es-ES/dashboard.json b/public/locales/es-ES/dashboard.json
index 99543791..9fe60778 100644
--- a/public/locales/es-ES/dashboard.json
+++ b/public/locales/es-ES/dashboard.json
@@ -267,6 +267,8 @@
     "fileIdVar": "ID de Archivo",
     "userIdVar": "ID de Usuario, vacío cuando no está conectado.",
     "userDisplayNameVar": "Nombre para mostrar del usuario codificado en URL.",
+    "themeVar": "Tema actual, \"light\" o \"dark\".",
+    "darkVar": "Si el tema oscuro está activo, \"1\" o \"0\".",
     "fileViewers": "Aplicaciones de archivos",
     "viewerList": "Lista de aplicaciones",
     "defaultViewerApps": "Aplicaciones predeterminadas",
diff --git a/public/locales/fr-FR/dashboard.json b/public/locales/fr-FR/dashboard.json
index 2e2c9938..856e1aff 100644
--- a/public/locales/fr-FR/dashboard.json
+++ b/public/locales/fr-FR/dashboard.json
@@ -267,6 +267,8 @@
     "fileIdVar": "ID de fichier",
     "userIdVar": "ID utilisateur, vide lorsque non connecté.",
     "userDisplayNameVar": "Nom d'affichage utilisateur encodé en URL.",
+    "themeVar": "Thème actuel, \"light\" ou \"dark\".",
+    "darkVar": "Indique si le thème sombre est actif, \"1\" ou \"0\".",
     "fileViewers": "Applications de fichier",
     "viewerList": "Liste des applications",
     "defaultViewerApps": "Applications par défaut",
diff --git a/public/locales/it-IT/dashboard.json b/public/locales/it-IT/dashboard.json
index 8e0615fb..30f3d1e0 100644
--- a/public/locales/it-IT/dashboard.json
+++ b/public/locales/it-IT/dashboard.json
@@ -267,6 +267,8 @@
     "fileIdVar": "ID File",
     "userIdVar": "ID Utente, vuoto quando non loggato.",
     "userDisplayNameVar": "Nome visualizzazione utente codificato URL.",
+    "themeVar": "Tema corrente, \"light\" o \"dark\".",
+    "darkVar": "Se il tema scuro è attivo, \"1\" o \"0\".",
     "fileViewers": "Applicazioni file",
     "viewerList": "Elenco applicazioni",
     "defaultViewerApps": "Applicazioni predefinite",
diff --git a/public/locales/ja-JP/dashboard.json b/public/locales/ja-JP/dashboard.json
index 7e48a63d..7e813701 100644
--- a/public/locales/ja-JP/dashboard.json
+++ b/public/locales/ja-JP/dashboard.json
@@ -266,6 +266,8 @@
     "fileIdVar": "ファイルID",
     "userIdVar": "ユーザーID（ログインしていない場合は空欄です）。",
     "userDisplayNameVar": "URLエンコード済みのユーザーニックネーム",
+    "themeVar": "現在のテーマ（\"light\" または \"dark\"）",
+    "darkVar": "ダークテーマが有効かどうか（\"1\" または \"0\"）",
     "fileViewers": "ファイル閲覧アプリ",
     "viewerList": "アプリ一覧",
     "defaultViewerApps": "デフォルトアプリ",
diff --git a/public/locales/ko-KR/dashboard.json b/public/locales/ko-KR/dashboard.json
index 6ca7123f..a3c76b09 100644
--- a/public/locales/ko-KR/dashboard.json
+++ b/public/locales/ko-KR/dashboard.json
@@ -266,6 +266,8 @@
     "fileIdVar": "파일 ID",
     "userIdVar": "사용자 ID, 로그인하지 않은 경우 비어있음",
     "userDisplayNameVar": "URL 인코딩된 사용자 닉네임",
+    "themeVar": "현재 테마, \"light\" 또는 \"dark\"",
+    "darkVar": "다크 테마 활성화 여부, \"1\" 또는 \"0\"",
     "fileViewers": "파일 브라우저 애플리케이션",
     "viewerList": "애플리케이션 목록",
     "defaultViewerApps": "기본 열기 방식",
diff --git a/public/locales/pl-PL/dashboard.json b/public/locales/pl-PL/dashboard.json
index 7b8ad4d0..6a6e246a 100644
--- a/public/locales/pl-PL/dashboard.json
+++ b/public/locales/pl-PL/dashboard.json
@@ -266,6 +266,8 @@
     "fileIdVar": "ID pliku",
     "userIdVar": "ID użytkownika, puste gdy nie jest zalogowany.",
     "userDisplayNameVar": "Zakodowana w URL wyświetlana nazwa użytkownika.",
+    "themeVar": "Bieżący motyw, \"light\" lub \"dark\".",
+    "darkVar": "Czy ciemny motyw jest aktywny, \"1\" lub \"0\".",
     "fileViewers": "Aplikacje plików",
     "viewerList": "Lista aplikacji",
     "defaultViewerApps": "Domyślne aplikacje",
diff --git a/public/locales/pt-BR/dashboard.json b/public/locales/pt-BR/dashboard.json
index 4b5245be..f998a5d0 100644
--- a/public/locales/pt-BR/dashboard.json
+++ b/public/locales/pt-BR/dashboard.json
@@ -267,6 +267,8 @@
     "fileIdVar": "ID do arquivo",
     "userIdVar": "ID do usuário, vazio quando não logado.",
     "userDisplayNameVar": "Nome de exibição do usuário codificado em URL.",
+    "themeVar": "Tema atual, \"light\" ou \"dark\".",
+    "darkVar": "Se o tema escuro está ativo, \"1\" ou \"0\".",
     "fileViewers": "Aplicações de arquivo",
     "viewerList": "Lista de aplicações",
     "defaultViewerApps": "Aplicações padrão",
diff --git a/public/locales/ru-RU/dashboard.json b/public/locales/ru-RU/dashboard.json
index b3a58181..847796e6 100644
--- a/public/locales/ru-RU/dashboard.json
+++ b/public/locales/ru-RU/dashboard.json
@@ -267,6 +267,8 @@
     "fileIdVar": "ID файла",
     "userIdVar": "ID пользователя, пустой при отсутствии входа в систему.",
     "userDisplayNameVar": "URL-кодированное отображаемое имя пользователя.",
+    "themeVar": "Текущая тема, \"light\" или \"dark\".",
+    "darkVar": "Активна ли тёмная тема, \"1\" или \"0\".",
     "fileViewers": "Файловые приложения",
     "viewerList": "Список приложений",
     "defaultViewerApps": "Приложения по умолчанию",
diff --git a/public/locales/zh-CN/dashboard.json b/public/locales/zh-CN/dashboard.json
index 6d02fa63..8e8a18cf 100644
--- a/public/locales/zh-CN/dashboard.json
+++ b/public/locales/zh-CN/dashboard.json
@@ -266,6 +266,8 @@
     "fileIdVar": "文件 ID",
     "userIdVar": "用户 ID，未登录时为空",
     "userDisplayNameVar": "经过 URL 编码后的用户昵称",
+    "themeVar": "当前主题，\"light\" 或 \"dark\"",
+    "darkVar": "是否为深色主题，\"1\" 或 \"0\"",
     "fileViewers": "文件浏览应用",
     "viewerList": "应用列表",
     "defaultViewerApps": "默认打开方式",
diff --git a/public/locales/zh-TW/dashboard.json b/public/locales/zh-TW/dashboard.json
index 3e7e436a..b7d58c50 100644
--- a/public/locales/zh-TW/dashboard.json
+++ b/public/locales/zh-TW/dashboard.json
@@ -266,6 +266,8 @@
     "fileIdVar": "檔案 ID",
     "userIdVar": "使用者 ID，未登入時為空。",
     "userDisplayNameVar": "經過 URL 編碼後的使用者暱稱。",
+    "themeVar": "目前主題，\"light\" 或 \"dark\"。",
+    "darkVar": "是否為深色主題，\"1\" 或 \"0\"。",
     "fileViewers": "檔案瀏覽應用",
     "viewerList": "應用列表",
     "defaultViewerApps": "預設開啟方式",
diff --git a/src/component/Admin/FileSystem/ViewerSetting/FileViewerEditDialog.tsx b/src/component/Admin/FileSystem/ViewerSetting/FileViewerEditDialog.tsx
index d6f30e23..beab4f48 100644
--- a/src/component/Admin/FileSystem/ViewerSetting/FileViewerEditDialog.tsx
+++ b/src/component/Admin/FileSystem/ViewerSetting/FileViewerEditDialog.tsx
@@ -93,6 +93,16 @@ const magicVars: MagicVar[] = [
     value: "settings.userDisplayNameVar",
     example: "Aaron%20Liu",
   },
+  {
+    name: "{$theme}",
+    value: "settings.themeVar",
+    example: "dark",
+  },
+  {
+    name: "{$dark}",
+    value: "settings.darkVar",
+    example: "1",
+  },
 ];
 
 const DND_TYPE = "template-row";
diff --git a/src/redux/thunks/viewer.ts b/src/redux/thunks/viewer.ts
index f652536b..4ecd68d4 100644
--- a/src/redux/thunks/viewer.ts
+++ b/src/redux/thunks/viewer.ts
@@ -289,7 +289,7 @@ export function openViewer(
 }
 
 export function openCustomViewer(file: FileResponse, viewer: Viewer, preferredVersion?: string): AppThunk {
-  return async (dispatch, _getState) => {
+  return async (dispatch, getState) => {
     const entityUrl = await longRunningTaskWithSnackbar(
       dispatch(
         getFileEntityUrl({
@@ -303,6 +303,14 @@ export function openCustomViewer(file: FileResponse, viewer: Viewer, preferredVe
 
     const currentUser = SessionManager.currentUser();
 
+    // Mirrors the fallback in useGeneratedTheme (App.tsx): explicit user
+    // preference wins, otherwise fall back to the OS/browser preference.
+    const darkModePreference = getState().globalState.darkMode;
+    const isDarkMode =
+      darkModePreference === undefined
+        ? window.matchMedia?.("(prefers-color-scheme: dark)").matches
+        : darkModePreference;
+
     const vars: { [key: string]: string } = {
       src: encodeURIComponent(entityUrl.urls[0].url),
       src_raw: entityUrl.urls[0].url,
@@ -312,6 +320,8 @@ export function openCustomViewer(file: FileResponse, viewer: Viewer, preferredVe
       id: file.id,
       user_id: currentUser?.id ?? "",
       user_display_name: encodeURIComponent(currentUser?.nickname ?? ""),
+      theme: isDarkMode ? "dark" : "light",
+      dark: isDarkMode ? "1" : "0",
     };
 
     // replace variables in viewer.url