I have a `MaterialApp` with a `ThemeData` that is initially set to `Brightness.light`. I'd like to switch the brightness to `Brightness.dark` at runtime, but when I make that change, only the status bar changes - none of the Flutter widgets actually change their brightness.

How can this behavior be achieved?

To change the `ThemeData` at runtime, I've created the following `StatefulWidget` that wraps my `MaterialApp` and rebuilds it whenever the theme changes:

```
final ThemeData appTheme = ThemeData(
  brightness: Brightness.light,
);

class ThemeChanger extends StatefulWidget {
  static ThemeChangerState of (BuildContext context) {
    return context.ancestorStateOfType(TypeMatcher<ThemeChangerState>());
  }

  ThemeChanger({
    this.childBuilder,
  });

  final Widget Function(BuildContext, ThemeData) childBuilder;

  @override
  ThemeChangerState createState() => ThemeChangerState();
}

class ThemeChangerState extends State<ThemeChanger> {
  Brightness _brightness = Brightness.light;

  set brightness(Brightness brightness) {
    setState(() {
      _brightness = brightness;
    });
  }

  @override
  Widget build(BuildContext context) {
    return widget.childBuilder(
      context,
      appTheme.copyWith(
        brightness: _brightness
      ),
    );
  }
}

// Then in main.dart
void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return DemoTheme(
      childBuilder: (BuildContext context, ThemeData theme) {
        return MaterialApp(
          title: 'Materially Better',
          theme: theme,
          routes: {
            '/': (BuildContext context) {
              return LoginScreen();
            },
            'home': (BuildContext context) {
              return MainScreen();
            }
          },
          debugShowCheckedModeBanner: false,
        );
      },
    );
  }
}

```