Overrides Drupal\Core\Entity\EntityRenderController::getBuildDefaults().
Overrides EntityRenderController::getBuildDefaults
protected function getBuildDefaults(EntityInterface $entity, $view_mode, $langcode) {
$defaults = parent::getBuildDefaults($entity, $view_mode, $langcode);
$defaults['#theme'] = 'aggregator_feed_source';
return $defaults;
}