Update Theme Setup (#2300)

* refactor(theme-setup-update): Move didChangePlatformBrightness lifecycle method to Root widget instead of AppState. We already have an active WidgetsBindingObserver in Root widget.

* fix(theme): Add workaround for flutter iOS didChangePlatformBrightness issue causing unneeded rebuilds.

lib/src/screens/root/root.dart

@@ -1,4 +1,5 @@
 import 'dart:async';
+import 'dart:io';
 import 'package:cake_wallet/bitcoin/bitcoin.dart';
 import 'package:cake_wallet/core/auth_service.dart';
 import 'package:cake_wallet/core/totp_request_details.dart';
@@ -159,6 +160,18 @@ class RootState extends State<Root> with WidgetsBindingObserver {
     }
   }
 
+  @override
+  void didChangePlatformBrightness() {
+    if (widget.appStore.themeStore.themeMode == ThemeMode.system) {
+      Future.delayed(Duration(milliseconds: Platform.isIOS ? 500 : 0), () {
+        final systemTheme = widget.appStore.themeStore.getThemeFromSystem();
+        if (widget.appStore.themeStore.currentTheme != systemTheme) {
+          widget.appStore.themeStore.setTheme(systemTheme);
+        }
+      });
+    }
+  }
+
   @override
   Widget build(BuildContext context) {
     // this only happens when the app has been in the background for some time