Main/Test

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Color Changer'),
        ),
        body: ColorChanger(),
      ),
    );
  }
}

class ColorChanger extends StatefulWidget {
  @override
  _ColorChangerState createState() => _ColorChangerState();
}

class _ColorChangerState extends State<ColorChanger> {
  Color _backgroundColor = Colors.white;

  void changeBackgroundColor() {
    setState(() {
      _backgroundColor = Color(Random().nextInt(0xFFFFFFFF));
    });
  }

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: () => changeBackgroundColor(),
      child: Container(
        color: _backgroundColor,
        child: Center(
          child: Text(
            'Tap to Change Color',
            style: TextStyle(fontSize: 20, fontWeight: FontWeight.bold),
          ),
        ),
      ),
    );
  }
}