diff --git a/core/modules/forum/forum.module b/core/modules/forum/forum.module
index 23293fe..01a6987 100644
--- a/core/modules/forum/forum.module
+++ b/core/modules/forum/forum.module
@@ -96,9 +96,6 @@ function forum_theme() {
       'template' => 'forum-submitted',
       'variables' => array('topic' => NULL),
     ),
-    'forum_form' => array(
-      'render element' => 'form',
-    ),
   );
 }
 
@@ -739,19 +736,3 @@ function template_preprocess_forum_submitted(&$variables) {
   }
   $variables['time'] = isset($variables['topic']->created) ? format_interval(REQUEST_TIME - $variables['topic']->created) : '';
 }
-
-/**
- * Returns HTML for a forum form.
- *
- * By default this does not alter the appearance of a form at all, but is
- * provided as a convenience for themers.
- *
- * @param $variables
- *   An associative array containing:
- *   - form: A render element representing the form.
- *
- * @ingroup themeable
- */
-function theme_forum_form(array $variables) {
-  return drupal_render_children($variables['form']);
-}
diff --git a/core/modules/forum/src/Form/ForumForm.php b/core/modules/forum/src/Form/ForumForm.php
index 34c64df..fc20134 100644
--- a/core/modules/forum/src/Form/ForumForm.php
+++ b/core/modules/forum/src/Form/ForumForm.php
@@ -53,7 +53,7 @@ public function form(array $form, array &$form_state) {
     $form['parent']['#tree'] = TRUE;
     $form['parent'][0] = $this->forumParentSelect($taxonomy_term->id(), $this->t('Parent'));
 
-    $form['#theme'] = 'forum_form';
+    $form['#theme_wrappers'] = array('form__forum');
     $this->forumFormType = $this->t('forum');
     return $form;
   }