store theme selection in localstorage

frontend/src/App.tsx

@@ -24,7 +24,13 @@ function App() {
   const [token, setToken] = useState<string>(
     localStorage.getItem("token") || ""
   );
-  const [theme, setTheme] = useState<string>("default");
+  const [theme, _setTheme] = useState<string>(
+    localStorage.getItem("theme") || "default"
+  );
+  const setTheme = (theme: string) => {
+    _setTheme(theme);
+    localStorage.setItem("theme", theme);
+  };
   const [toasts, setToasts] = useState<ToastMessage[]>([]);
   const [isLoading, setIsLoading] = useState(false);