diff --git a/core/modules/forum/forum.module b/core/modules/forum/forum.module
index 1cbfa85..b046ea9 100644
--- a/core/modules/forum/forum.module
+++ b/core/modules/forum/forum.module
@@ -1065,7 +1065,8 @@ function template_preprocess_forum_list(&$variables) {
       }
       $variables['forums'][$id]->old_topics = $forum->num_topics - $variables['forums'][$id]->new_topics;
     }
-    $variables['forums'][$id]->last_reply = theme('forum_submitted', array('topic' => $forum->last_post));
+    $forum_submitted = array('#theme' => 'forum_submitted', '#topic' => $forum->last_post);
+    $variables['forums'][$id]->last_reply = drupal_render($forum_submitted);
   }
 
   $variables['pager'] = array(
@@ -1127,8 +1128,13 @@ function template_preprocess_forum_topic_list(&$variables) {
         $variables['topics'][$id]->title = l($topic->title, "node/$topic->nid");
         $variables['topics'][$id]->message = '';
       }
-      $variables['topics'][$id]->created = theme('forum_submitted', array('topic' => $topic));
-      $variables['topics'][$id]->last_reply = theme('forum_submitted', array('topic' => isset($topic->last_reply) ? $topic->last_reply : NULL));
+      $forum_submitted = array('#theme' => 'forum_submitted', '#topic' => $topic);
+      $variables['topics'][$id]->created = drupal_render($forum_submitted);
+      $last_reply = array(
+        '#theme' => 'forum_submitted',
+        '#topic' => isset($topic->last_reply) ? $topic->last_reply : NULL,
+      );
+      $variables['topics'][$id]->last_reply = drupal_render($last_reply);
 
       $variables['topics'][$id]->new_text = '';
       $variables['topics'][$id]->new_url = '';
@@ -1206,7 +1212,11 @@ function template_preprocess_forum_icon(&$variables) {
  *   - topic: The topic object.
  */
 function template_preprocess_forum_submitted(&$variables) {
-  $variables['author'] = isset($variables['topic']->uid) ? theme('username', array('account' => user_load($variables['topic']->uid))) : '';
+  $variables['author'] = '';
+  if (isset($variables['topic']->uid)) {
+    $username = array('#theme' => 'username', '#account' => user_load($variables['topic']->uid));
+    $variables['author'] = drupal_render($username);
+  }
   $variables['time'] = isset($variables['topic']->created) ? format_interval(REQUEST_TIME - $variables['topic']->created) : '';
 }
 
diff --git a/core/modules/forum/lib/Drupal/forum/Tests/ForumTest.php b/core/modules/forum/lib/Drupal/forum/Tests/ForumTest.php
index 8297e80..2b47913 100644
--- a/core/modules/forum/lib/Drupal/forum/Tests/ForumTest.php
+++ b/core/modules/forum/lib/Drupal/forum/Tests/ForumTest.php
@@ -573,13 +573,17 @@ private function verifyForums($node_user, EntityInterface $node, $admin, $respon
     $this->drupalGet('node/' . $node->nid);
     $this->assertResponse(200);
     $this->assertTitle($node->label() . ' | Drupal', 'Forum node was displayed');
-    $breadcrumb = array(
+    $breadcrumb_build = array(
       l(t('Home'), NULL),
       l(t('Forums'), 'forum'),
       l($this->forumContainer['name'], 'forum/' . $this->forumContainer['tid']),
       l($this->forum['name'], 'forum/' . $this->forum['tid']),
     );
-    $this->assertRaw(theme('breadcrumb', array('breadcrumb' => $breadcrumb)), 'Breadcrumbs were displayed');
+    $breadcrumb = array(
+      '#theme' => 'breadcrumb',
+      '#breadcrumb' => $breadcrumb_build,
+    );
+    $this->assertRaw(drupal_render($breadcrumb), 'Breadcrumbs were displayed');
 
     // View forum edit node.
     $this->drupalGet('node/' . $node->nid . '/edit');
@@ -628,15 +632,19 @@ private function verifyForumView($forum, $parent = NULL) {
     $this->assertResponse(200);
     $this->assertTitle($forum['name'] . ' | Drupal');
 
-    $breadcrumb = array(
+    $breadcrumb_build = array(
       l(t('Home'), NULL),
       l(t('Forums'), 'forum'),
     );
     if (isset($parent)) {
-      $breadcrumb[] = l($parent['name'], 'forum/' . $parent['tid']);
+      $breadcrumb_build[] = l($parent['name'], 'forum/' . $parent['tid']);
     }
 
-    $this->assertRaw(theme('breadcrumb', array('breadcrumb' => $breadcrumb)));
+    $breadcrumb = array(
+      '#theme' => 'breadcrumb',
+      '#breadcrumb' => $breadcrumb_build,
+    );
+    $this->assertRaw(drupal_render($breadcrumb), 'Breadcrumbs were displayed');
   }
 
   /**