code: public function getStylesheets(GetOptionsEvent $event)
      {
          if (!$this->wantToHandle($event)
              || ($event->getPropertyName() !== 'additionalCss')
              || ($event->getSubPropertyName() !== 'file')) {
              return;
          }
  
          $event->setOptions($this->scanFiles('css'));
      }
docs: Provide options for additional css files.
  
  @param GetOptionsEvent $event The event.
  
  @return void