Simple Theme Picker

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 ListenableBuilder(
      listenable: controller,
      builder: (context, child) => MaterialApp(
        title: 'Flutter Demo',
        theme: ThemeData(colorScheme: controller.scheme),
        home: const ThemePickerPage(),
      ),
    );
  }
}

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: ListView(
        children: [
          for (final scheme in controller.options)
            ColorSchemeViewButton(scheme: scheme),
        ],
      ),
    );
  }
}

class ColorSchemeViewButton extends StatelessWidget {
  const ColorSchemeViewButton({
    super.key,
    required this.scheme,
  });

  final ColorScheme scheme;

  @override
  Widget build(BuildContext context) {
    return ListenableBuilder(
      listenable: controller,
      builder: (context, _) {
        return Theme(
          data: ThemeData(colorScheme: scheme),
          child: Builder(
            builder: (context) {
              final colors = Theme.of(context).colorScheme;
              return InkWell(
                onTap: () => controller.setScheme(scheme),
                child: Row(
                  mainAxisAlignment: MainAxisAlignment.center,
                  children: [
                    Visibility.maintain(
                      visible: scheme == controller.scheme,
                      child: const Icon(Icons.check_circle_sharp),
                    ),
                    Padding(
                      padding: const EdgeInsets.all(8.0),
                      child: SizedBox.square(
                        dimension: 20,
                        child: ColoredBox(color: colors.primary),
                      ),
                    ),
                    Padding(
                      padding: const EdgeInsets.all(8.0),
                      child: SizedBox.square(
                        dimension: 20,
                        child: ColoredBox(color: colors.secondary),
                      ),
                    ),
                    Padding(
                      padding: const EdgeInsets.all(8.0),
                      child: SizedBox.square(
                        dimension: 20,
                        child: ColoredBox(color: colors.tertiary),
                      ),
                    ),
                    Padding(
                      padding: const EdgeInsets.all(8.0),
                      child: SizedBox.square(
                        dimension: 20,
                        child: ColoredBox(color: colors.error),
                      ),
                    )
                  ],
                ),
              );
            },
          ),
        );
      },
    );
  }
}

final controller = ColorSchemeController();

class ColorSchemeController extends ChangeNotifier {
  static final _defaultScheme = ColorScheme.fromSeed(seedColor: Colors.blue);

  var scheme = _defaultScheme;

  final options = <ColorScheme>[
    _defaultScheme,
    ColorScheme.fromSeed(seedColor: Colors.red),
    ColorScheme.fromSeed(seedColor: Colors.yellow),
    ColorScheme.fromSeed(seedColor: Colors.green),
  ];

  void setScheme(ColorScheme colorScheme) {
    scheme = colorScheme;
    notifyListeners();
  }
}