adaptive_theme.dart

class Adaptation<T> {
  const Adaptation();
  Type get type => T;
  T adapt(ThemeData theme, T defaultValue) => defaultValue;
}

class ThemeData {
  factory ThemeData({ Iterable<Adaptation>?  adaptations }) {
    return ThemeData.raw(
      adaptationMap: <Type, Adaptation>{
        for (Adaptation adaptation in adaptations ?? const <Adaptation>[])
          adaptation.type: adaptation
        },
      );
  }
                                                         
  const ThemeData.raw({ required this.adaptationMap });

  final Map<Type, Adaptation> adaptationMap;

  T adaptive<T>(T defaultValue) {
    final Adaptation? adaptation = adaptationMap[T];
    return adaptation == null ? defaultValue : adaptation.adapt(this, defaultValue);
  }
}

class StringAdaptation extends Adaptation<String>{
  const StringAdaptation();
  @override
  String adapt(ThemeData theme, String defaultValue) => 'I am an adaptive theme';
}

void main() {
  final ThemeData myTheme = ThemeData(
    adaptations: [const StringAdaptation()],
  );
    
  print(myTheme.adaptive<String>('I am the default theme'));
}