add macos, desktop

filcnaplo/lib/theme/observer.dart

@@ -0,0 +1,13 @@
+import 'package:flutter/material.dart';
+
+class ThemeModeObserver extends ChangeNotifier {
+  ThemeMode _themeMode;
+  ThemeMode get themeMode => _themeMode;
+
+  ThemeModeObserver({ThemeMode initialTheme = ThemeMode.system}) : _themeMode = initialTheme;
+
+  void changeTheme(ThemeMode mode) {
+    _themeMode = mode;
+    notifyListeners();
+  }
+}