diff --git a/index.html b/index.html
index 58d5cfa..cdecdcf 100644
--- a/index.html
+++ b/index.html
@@ -4,8 +4,14 @@
     <meta charset="UTF-8" />
     <script>
       (function () {
-        var key = 'memind-theme';
-        var stored = localStorage.getItem(key);
+        var storageKey = 'memind-theme';
+        var migrationKey = 'memind-theme-migration';
+        var migrationVersion = 'dark-default-2026-09-11';
+        if (localStorage.getItem(migrationKey) !== migrationVersion) {
+          localStorage.setItem(storageKey, 'dark');
+          localStorage.setItem(migrationKey, migrationVersion);
+        }
+        var stored = localStorage.getItem(storageKey);
         var theme = stored === 'light' || stored === 'dark' ? stored : 'dark';
         document.documentElement.dataset.theme = theme;
         document.documentElement.style.colorScheme = theme;
diff --git a/public/oa-drive/app.js b/public/oa-drive/app.js
index 20a337c..9376b7a 100644
--- a/public/oa-drive/app.js
+++ b/public/oa-drive/app.js
@@ -2,6 +2,14 @@
   'use strict';
 
   const THEME_STORAGE_KEY = 'memind-theme';
+  const THEME_MIGRATION_KEY = 'memind-theme-migration';
+  const THEME_MIGRATION_VERSION = 'dark-default-2026-09-11';
+
+  function migrateThemePreferenceIfNeeded() {
+    if (localStorage.getItem(THEME_MIGRATION_KEY) === THEME_MIGRATION_VERSION) return;
+    localStorage.setItem(THEME_STORAGE_KEY, 'dark');
+    localStorage.setItem(THEME_MIGRATION_KEY, THEME_MIGRATION_VERSION);
+  }
 
   function getStoredTheme() {
     const stored = localStorage.getItem(THEME_STORAGE_KEY);
@@ -32,6 +40,7 @@
   }
 
   function initTheme() {
+    migrateThemePreferenceIfNeeded();
     applyTheme(getStoredTheme());
     updateThemeToggleLabel();
     window.addEventListener('storage', (event) => {
diff --git a/src/utils/theme.ts b/src/utils/theme.ts
index 6378521..9f8a267 100644
--- a/src/utils/theme.ts
+++ b/src/utils/theme.ts
@@ -1,6 +1,15 @@
 export type MemindTheme = 'light' | 'dark';
 
 export const THEME_STORAGE_KEY = 'memind-theme';
+export const THEME_MIGRATION_KEY = 'memind-theme-migration';
+export const THEME_MIGRATION_VERSION = 'dark-default-2026-09-11';
+
+export function migrateThemePreferenceIfNeeded() {
+  if (typeof localStorage === 'undefined') return;
+  if (localStorage.getItem(THEME_MIGRATION_KEY) === THEME_MIGRATION_VERSION) return;
+  localStorage.setItem(THEME_STORAGE_KEY, 'dark');
+  localStorage.setItem(THEME_MIGRATION_KEY, THEME_MIGRATION_VERSION);
+}
 
 export function getStoredTheme(): MemindTheme {
   const stored = localStorage.getItem(THEME_STORAGE_KEY);
@@ -17,6 +26,7 @@ export function applyTheme(theme: MemindTheme) {
 }
 
 export function initTheme() {
+  migrateThemePreferenceIfNeeded();
   applyTheme(getStoredTheme());
 }