diff --git a/core/modules/comment/comment.admin.inc b/core/modules/comment/comment.admin.inc
index 488c2f9..2d5814e 100644
--- a/core/modules/comment/comment.admin.inc
+++ b/core/modules/comment/comment.admin.inc
@@ -39,7 +39,6 @@ function comment_admin($type = 'new') {
  * @see comment_admin()
  * @see comment_admin_overview_validate()
  * @see comment_admin_overview_submit()
- * @see theme_comment_admin_overview()
  */
 function comment_admin_overview($form, &$form_state, $arg) {
   // Build an 'Update options' form.
@@ -119,7 +118,12 @@ function comment_admin_overview($form, &$form_state, $arg) {
           '#options' => array('attributes' => array('title' => truncate_utf8($comment->comment_body->value, 128)), 'fragment' => 'comment-' . $comment->id()),
         ),
       ),
-      'author' => theme('username', array('account' => comment_prepare_author($comment))),
+      'author' => array(
+        'data' => array(
+          '#theme' => 'username',
+          '#account' => comment_prepare_author($comment)
+        ),
+      ),
       'posted_in' => array(
         'data' => array(
           '#type' => 'link',
diff --git a/core/modules/comment/comment.module b/core/modules/comment/comment.module
index a4c7a42..66328df 100644
--- a/core/modules/comment/comment.module
+++ b/core/modules/comment/comment.module
@@ -186,18 +186,13 @@ function comment_field_extra_fields() {
 function comment_theme() {
   return array(
     'comment_block' => array(
+      'template' => 'comment-block',
       'variables' => array('number' => NULL),
     ),
-    'comment_preview' => array(
-      'variables' => array('comment' => NULL),
-    ),
     'comment' => array(
       'template' => 'comment',
       'render element' => 'elements',
     ),
-    'comment_post_forbidden' => array(
-      'variables' => array('node' => NULL),
-    ),
     'comment_wrapper' => array(
       'template' => 'comment-wrapper',
       'render element' => 'content',
@@ -540,26 +535,6 @@ function comment_new_page_count($num_comments, $new_replies, EntityInterface $no
 }
 
 /**
- * Returns HTML for a list of recent comments.
- *
- * @ingroup themeable
- */
-function theme_comment_block($variables) {
-  $items = array();
-  $number = $variables['number'];
-  foreach (comment_get_recent($number) as $comment) {
-    $items[] = l($comment->subject, 'comment/' . $comment->cid, array('fragment' => 'comment-' . $comment->cid)) . '&nbsp;<span>' . t('@time ago', array('@time' => format_interval(REQUEST_TIME - $comment->changed))) . '</span>';
-  }
-
-  if ($items) {
-    return theme('item_list', array('items' => $items));
-  }
-  else {
-    return t('No comments available.');
-  }
-}
-
-/**
  * Implements hook_node_view().
  */
 function comment_node_view(EntityInterface $node, EntityDisplay $display, $view_mode) {
@@ -614,7 +589,7 @@ function comment_node_view(EntityInterface $node, EntityDisplay $display, $view_
         }
         else {
           $links['comment-forbidden'] = array(
-            'title' => theme('comment_post_forbidden', array('node' => $node)),
+            'title' => comment_forbidden_message($node),
             'html' => TRUE,
           );
         }
@@ -644,7 +619,7 @@ function comment_node_view(EntityInterface $node, EntityDisplay $display, $view_
         }
         else {
           $links['comment-forbidden'] = array(
-            'title' => theme('comment_post_forbidden', array('node' => $node)),
+            'title' => comment_forbidden_message($node),
             'html' => TRUE,
           );
         }
@@ -946,7 +921,7 @@ function comment_links(Comment $comment, EntityInterface $node) {
       );
     }
     if (empty($links)) {
-      $links['comment-forbidden']['title'] = theme('comment_post_forbidden', array('node' => $node));
+      $links['comment-forbidden']['title'] = comment_forbidden_message($node);
       $links['comment-forbidden']['html'] = TRUE;
     }
   }
@@ -1554,6 +1529,28 @@ function comment_prepare_author(Comment $comment) {
 }
 
 /**
+ * Prepares variables for comment block templates.
+ *
+ * Default template: comment-block.html.twig.
+ *
+ * @param array $variables
+ *   An associative array containing:
+ *   - number: The number of recent comments to display.
+ */
+function template_preprocess_comment_block(&$variables) {
+  $items = array();
+  foreach (comment_get_recent($variables['number']) as $comment) {
+    $items[] = l($comment->subject, 'comment/' . $comment->cid, array('fragment' => 'comment-' . $comment->cid)) . '&nbsp;<span>' . t('@time ago', array('@time' => format_interval(REQUEST_TIME - $comment->changed))) . '</span>';
+  }
+  if (!empty($items)) {
+    $variables['comments'] = array(
+      '#theme' => 'item_list',
+      '#items' => $items,
+    );
+  }
+}
+
+/**
  * Preprocesses variables for comment.tpl.php.
  *
  * @see comment.tpl.php
@@ -1667,50 +1664,48 @@ function template_preprocess_comment(&$variables) {
 }
 
 /**
- * Returns HTML for a "you can't post comments" notice.
+ * Provides a message if posting comments is forbidden.
  *
- * @param $variables
- *   An associative array containing:
- *   - node: The comment node.
+ * If authenticated users can post comments, a message is returned that prompts
+ * the anonymous user to log in (or register, if applicable) that redirects to
+ * node comment form. Otherwise, no message is returned.
  *
- * @ingroup themeable
+ * @param \Drupal\Core\Entity\EntityInterface $node
+ *   The node the comment is attached to.
  */
-function theme_comment_post_forbidden($variables) {
-  $node = $variables['node'];
-  global $user;
+function comment_forbidden_message(EntityInterface $node) {
 
+  // Whether the user will be able to post comments once logged in.
   // Since this is expensive to compute, we cache it so that a page with many
   // comments only has to query the database once for all the links.
-  $authenticated_post_comments = &drupal_static(__FUNCTION__, NULL);
-
-  if (!$user->uid) {
-    if (!isset($authenticated_post_comments)) {
-      // We only output a link if we are certain that users will get permission
-      // to post comments by logging in.
-      $comment_roles = user_roles(TRUE, 'post comments');
-      $authenticated_post_comments = isset($comment_roles[DRUPAL_AUTHENTICATED_RID]);
-    }
-
-    if ($authenticated_post_comments) {
-      // We cannot use drupal_get_destination() because these links
-      // sometimes appear on /node and taxonomy listing pages.
-      if (variable_get('comment_form_location_' . $node->type, COMMENT_FORM_BELOW) == COMMENT_FORM_SEPARATE_PAGE) {
-        $destination = array('destination' => "comment/reply/$node->nid#comment-form");
-      }
-      else {
-        $destination = array('destination' => "node/$node->nid#comment-form");
-      }
+  $auth_post_comments = &drupal_static(__FUNCTION__, NULL);
+  if (!isset($auth_post_comments)) {
+    $comment_roles = user_roles(TRUE, 'post comments');
+    $auth_post_comments = isset($comment_roles[DRUPAL_AUTHENTICATED_RID]);
+  }
 
-      if (config('user.settings')->get('register') != USER_REGISTER_ADMINISTRATORS_ONLY) {
-        // Users can register themselves.
-        return t('<a href="@login">Log in</a> or <a href="@register">register</a> to post comments', array('@login' => url('user/login', array('query' => $destination)), '@register' => url('user/register', array('query' => $destination))));
-      }
-      else {
-        // Only admins can add new users, no public registration.
-        return t('<a href="@login">Log in</a> to post comments', array('@login' => url('user/login', array('query' => $destination))));
-      }
+  if (!user_is_logged_in() && $auth_post_comments) {
+    // Build login and register links. We cannot use drupal_get_destination()
+    // because these links sometimes appear on /node and taxonomy listing pages.
+    if (variable_get('comment_form_location_' . $node->type, COMMENT_FORM_BELOW) == COMMENT_FORM_SEPARATE_PAGE) {
+      $destination = array('destination' => "comment/reply/$node->nid#comment-form");
+    }
+    else {
+      $destination = array('destination' => "node/$node->nid#comment-form");
+    }
+    // Message depends on whether users may register accounts.
+    if (config('user.settings')->get('register') != USER_REGISTER_ADMINISTRATORS_ONLY) {
+      $message = t('<a href="@login">Log in</a> or <a href="@register">register</a> to post comments', array('@login' => url('user/login', array('query' => $destination)), '@register' => url('user/register', array('query' => $destination))));
+    }
+    else {
+      $message = t('<a href="@login">Log in</a> to post comments', array('@login' => url('user/login', array('query' => $destination))));
     }
   }
+  else {
+    $message = '';
+  }
+
+  return $message;
 }
 
 /**
diff --git a/core/modules/comment/lib/Drupal/comment/CommentFormController.php b/core/modules/comment/lib/Drupal/comment/CommentFormController.php
index 179410c..1ce51be 100644
--- a/core/modules/comment/lib/Drupal/comment/CommentFormController.php
+++ b/core/modules/comment/lib/Drupal/comment/CommentFormController.php
@@ -91,7 +91,8 @@ public function form(array $form, array &$form_state) {
     elseif ($user->uid) {
       $form['author']['name']['#type'] = 'item';
       $form['author']['name']['#value'] = $form['author']['name']['#default_value'];
-      $form['author']['name']['#markup'] = theme('username', array('account' => $user));
+      $form['author']['name']['#theme'] = 'username';
+      $form['author']['name']['#account'] = $user;
     }
 
     // Add author e-mail and homepage fields depending on the current user.
diff --git a/core/modules/comment/lib/Drupal/comment/Tests/CommentLinksTest.php b/core/modules/comment/lib/Drupal/comment/Tests/CommentLinksTest.php
index 5585650..d3eba8a 100644
--- a/core/modules/comment/lib/Drupal/comment/Tests/CommentLinksTest.php
+++ b/core/modules/comment/lib/Drupal/comment/Tests/CommentLinksTest.php
@@ -260,7 +260,7 @@ function assertCommentLinks(array $info) {
 
         // Anonymous users should see a note to log in or register in case
         // authenticated users are allowed to post comments.
-        // @see theme_comment_post_forbidden()
+        // @see comment-post-forbidden.html.twig
         if (!$this->loggedInUser) {
           if (user_access('post comments', $this->web_user)) {
             // The note depends on whether users are actually able to register.
diff --git a/core/modules/comment/templates/comment-block.html.twig b/core/modules/comment/templates/comment-block.html.twig
new file mode 100644
index 0000000..2ec3d00
--- /dev/null
+++ b/core/modules/comment/templates/comment-block.html.twig
@@ -0,0 +1,19 @@
+{#
+/**
+ * @file
+ * Default theme implementation for a list of recent comments.
+ *
+ * Available variables:
+ * - comments: A list of recent comments.
+ *
+ * @see template_preprocess()
+ * @see template_preprocess_comment_block()
+ *
+ * @ingroup themeable
+ */
+#}
+{% if not comments %}
+  {{ 'No comments available.'|t }}
+{% else %}
+  {{ comments }}
+{% endif %}
