Author: Kamil Kokot (kamil.kokot@lakion.com)
Inheritance: extends Symfony\Component\DependencyInjection\Extension\Extension, implements Symfony\Component\DependencyInjection\Extension\PrependExtensionInterface
 /**
  * {@inheritdoc}
  */
 protected function getContainerExtensions()
 {
     $themeExtension = new SyliusThemeExtension();
     $themeExtension->addConfigurationSourceFactory(new FilesystemConfigurationSourceFactory());
     return [$themeExtension];
 }