Implements \Drupal\Core\Form\FormInterface::submitForm().
Overrides SystemConfigFormBase::submitForm
public function submitForm(array &$form, array &$form_state) {
$this->configFactory
->get('forum.settings')
->set('topics.hot_threshold', $form_state['values']['forum_hot_topic'])
->set('topics.page_limit', $form_state['values']['forum_per_page'])
->set('topics.order', $form_state['values']['forum_order'])
->save();
parent::submitForm($form, $form_state);
}