/**
* Adds theme configuration fields to main theme edit form
*
* @param FormEvent $event
*/
public function onThemeFormInit(FormEvent $event)
{
$builder = $event->getFormBuilder();
$resource = $event->getEntity();
$fieldGenerator = $this->container->get('theme.fields_generator');
$fieldGenerator->loadThemeFieldsConfiguration($resource);
$fieldGenerator->addFormFields($builder);
}