diff --git a/packages/core/src/theme/ThemeManager.ts b/packages/core/src/theme/ThemeManager.ts
index c78f3ee..39dfba6 100644
--- a/packages/core/src/theme/ThemeManager.ts
+++ b/packages/core/src/theme/ThemeManager.ts
@@ -13,7 +13,7 @@ export type ThemeManagerEvents = "themechange";
 
 export class ThemeManager extends TinyEmitter {
   private _theme: Theme;
-  private _storage: any;
+  private _storage: Storage;
   private _themes: Theme[];
   // The first theme is the default.
   constructor(storage: Storage, themeList: Theme[] = themes) {
@@ -34,7 +34,7 @@ export class ThemeManager extends TinyEmitter {
   }
 
   private writetToLocalStorage(value: Theme) {
-    this._storage.set(THEME_ID, value.id);
+    this._storage.setString(THEME_ID, value.id);
   }
 
   private readFromLocalStorage(): Theme | undefined {