How to apply CSS classes to another component in AngularDart?

Let's say there is a simple framework to display popups:

@Component(
  selector: 'popup-host',
  template: '''
      <div class="popup-container">
        <ng-template #popupRef></ng-template>
      </div>
  ''',
  styles: ['.popup-container { position: absolute; top: 100; left: 100; z-index: 100; }'],
)
class PopupContainerComponent {
  final PopupController _controller;
  final ComponentLoader _loader;

  PopupContainerComponent(this._controller, this._loader);

  void ngOnInit() {
    _controller.container = this;
  }

  @ViewChild('popupRef', read: ComponentRef)
  ComponentRef popupRef;

  void render(PopupConfig config) {
    final componentRef = _loader.loadNextTo(config.factory, popupRef);
    if (componentRef.instance is HasValueSetter) {
      componentRef.instance.value = config.value;
    }
  }
}

@Injectable()
class PopupController {
  PopupContainerComponent _container;
  set container(PopupContainerComponent container) => _container = container;

  void showPopup(PopupConfig config) {
    container.render(config);
  }
  ...
}

class PopupConfig {
  final ComponentFactory factory;
  final dynamic value;
  PopupConfig(this.factory, [this.value]);
}

abstract class HasValueSetter {
  set value(dynamic value);
}

This can then be used like this:

// Somewhere in the root template
<popup-host></popup-host>

// In popup.dart
@Component(
  selector: 'happy-popup',
  template: '''
      <div class="header">This is the popup content.</div>
      <div class="main">The value is .</div>
      <div class="footer">I am happy!</div>
  ''',
)
class HappyPopupComponent implements HasValueSetter {
  @override
  dynamic value;
}

// In some_other.dart
@Component(
  ...
  styles: [
    '.header { font-weight: bold }',
    '.main { color: red }',
    '.footer { color: green; font-style: italic }',
  ],
  ...
)
class SomeOtherComponent {
  final PopupController _popupController;
  ...
  SomeOtherComponent(this._popupController, ...) ...;

  void displayPopup() {
    _popupController.showPopup(HappyPopupComponentNgFactory, 42);
  }
}
...

Is there a way to forward styles from <some-other-component> to <happy-popup> without having to define them at the root of the app?

from How to apply CSS classes to another component in AngularDart?