diff --git a/client/src/util/storage.ts b/client/src/util/storage.ts
index a119291..d80a3f3 100644
--- a/client/src/util/storage.ts
+++ b/client/src/util/storage.ts
@@ -1,5 +1,5 @@
 import type { JsonWebToken, SavedOAuthToken, UserTheme } from "@fumix/fu-blog-common";
-import { base64UrlToBuffer, isOAuthType } from "@fumix/fu-blog-common";
+import { base64UrlToBuffer, isOAuthType, isUserTheme } from "@fumix/fu-blog-common";
 
 type OAuthState = { key: string; redirect_uri?: string };
 const idTokenKey = "id_token";
@@ -67,7 +67,17 @@ export function saveCssPreference(css: UserTheme) {
 }
 
 export function loadCssPreference(): UserTheme {
-  return loadFromStorageAsString(window.localStorage, "cssTheme", "lightTheme") as UserTheme;
+  return loadFromStorage(
+    window.localStorage,
+    "cssTheme",
+    (saved) => {
+      if (isUserTheme(saved)) {
+        return saved;
+      }
+      return null;
+    },
+    "light",
+  );
 }
 
 //
diff --git a/common/src/entity/User.ts b/common/src/entity/User.ts
index da713b9..a51d7f5 100644
--- a/common/src/entity/User.ts
+++ b/common/src/entity/User.ts
@@ -28,4 +28,9 @@ export type UserWithOAuthProviders = User & {
   oauthProviders: OAuthProviderId[];
 };
 
-export type UserTheme = "light" | "dark";
+const userThemes = ["light", "dark"] as const;
+export type UserTheme = (typeof userThemes)[number];
+
+export function isUserTheme(s: string): s is UserTheme {
+  return userThemes.some((it) => it === s);
+}