main.dart

typedef AdaptiveThemeCallback = Object? Function(ThemeData theme, Type type, Object? defaultValue);

Object? defaultCallback(ThemeData theme, Type type, Object? defaultValue) => defaultValue;

class ThemeData {
  factory ThemeData({ AdaptiveThemeCallback? callback }) {
    return ThemeData.raw(callback: callback ?? defaultCallback);
  }

  const ThemeData.raw({ this.callback = defaultCallback });

  final AdaptiveThemeCallback callback;

  T adaptive<T>(T defaultValue) => callback(this, T, defaultValue) as T;
}

Object? myCallback(ThemeData theme, Type type, Object? defaultValue) {
  return (type == String ? 'I am an adaptive theme' : defaultValue);
}

void main() {
  // Unfortunately, at this point myCallback's type parameters become `dynamic`
  final ThemeData myTheme = ThemeData(callback: myCallback);
 
  // Foo.adaptive gets its version of the theme like this:
  print(myTheme.adaptive<String>('I am a theme'));
}