switching theme without any library.

main.dart

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

/// Properties that help me keep track of the example being run.
bool _useMaterial = false;

class _SwitchingThemeApp extends StatefulWidget {
  @override
  _SwitchingThemeAppState createState() => _SwitchingThemeAppState();

  static Future<bool> updateTheme(
    final BuildContext context,
    final ThemeMode themeMode,
  ) async {
    final state = context.ancestorStateOfType(
      const TypeMatcher<_SwitchingThemeAppState>(),
    ) as _SwitchingThemeAppState;

    if (state?.mounted == true) {
      state.updateTheme(themeMode);
      return true;
    }
    return false;
  }
}

class _SwitchingThemeAppState extends State<_SwitchingThemeApp> {
  ThemeMode _themeMode = ThemeMode.system;
  final ThemeData _light = ThemeData.from(colorScheme: ColorScheme.light());
  final ThemeData _dark = ThemeData.from(colorScheme: ColorScheme.dark());

  void updateTheme(ThemeMode mode) {
    setState(() {
      _themeMode = mode;
    });
  }

  @override
  Widget build(BuildContext context) {
    if (_useMaterial) {
      return MaterialApp(
        home: _DummyPage(),
        themeMode: _themeMode,
        theme: _light,
        darkTheme: _dark,
      );
    } else {
      final platformBrightness = MediaQuery.platformBrightnessOf(context);

      ThemeData currentTheme;

      if (_themeMode == ThemeMode.system) {
        currentTheme = platformBrightness == Brightness.light ? _light : _dark;
      } else {
        currentTheme = _themeMode == ThemeMode.light ? _light : _dark;
      }

      return CupertinoApp(
        home: _DummyPage(),
        theme: MaterialBasedCupertinoThemeData(materialTheme: currentTheme),
      );
    }
  }
}

class _DummyPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    /// Required only because we don't if the example
    /// is being run using material widget or cupertino widget.
    final messageStyle = _useMaterial
        ? Theme.of(context).textTheme.headline
        : CupertinoTheme.of(context).textTheme.navLargeTitleTextStyle;

    final background = _useMaterial
        ? Theme.of(context).scaffoldBackgroundColor
        : CupertinoTheme.of(context).scaffoldBackgroundColor;

    final brightness = _useMaterial
        ? Theme.of(context).brightness
        : CupertinoTheme.of(context).brightness;

    return SafeArea(
      child: Container(
        color: background,
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              "My color will change depending on the theme",
              style: messageStyle,
              textAlign: TextAlign.center,
            ),
            Switch.adaptive(
              value: brightness == Brightness.dark,
              onChanged: (enable) {
                _SwitchingThemeApp.updateTheme(
                  context,
                  enable ? ThemeMode.dark : ThemeMode.light,
                );
              },
            )
          ],
        ),
      ),
    );
  }
}