Fix local/session storage parsing/stringifying values

The old logic stringifyed all values, including strings, which caused problems with reading external stored values (e.g., mui-mode) via the "useStorage" hook.

index.html

@@ -30,10 +30,11 @@
 <script type="module" src="./src/index.tsx"></script>
 <script>
     const backgroundColor = (() => {
+        const storageValue = window.localStorage.getItem("theme_background")
         try {
-            return JSON.parse(window.localStorage.getItem("theme_background"))
+            return JSON.parse(storageValue)
         } catch (e) {
-            return null
+            return storageValue
         }
     })();