main.dart

import 'package:flutter/material.dart';

void main() => runApp(const MyApp());

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      home: StatefulBuilder(builder: (_, setState) {
        onChange() {
          MyTheme.instance =
              MyTheme.instance == MyTheme.light ? MyTheme.dark : MyTheme.light;
          setState(() {});
        }

        return Scaffold(
          backgroundColor: MyTheme.instance.bgColor,
          body: GestureDetector(
            onTap: onChange,
            behavior: HitTestBehavior.opaque,
            child: Center(
              child: Text(
                "Change Theme",
                style: TextStyle(color: MyTheme.instance.color),
              ),
            ),
          ),
        );
      }),
    );
  }
}

class MyTheme {
  static var instance = light;

  static var light = MyTheme();
  static var dark = MyTheme()
    ..color = Colors.white
    ..bgColor = Colors.black;

  var color = Colors.black;
  var bgColor = Colors.white;
}