diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 8eafe76..6b2e454 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -3220,11 +3220,9 @@ function drupal_common_theme() {
     ),
     'update_page' => array(
       'variables' => array('content' => NULL, 'show_messages' => TRUE),
-      'base hook' => 'maintenance_page',
     ),
     'install_page' => array(
-      'variables' => array('content' => NULL, 'show_messages' => NULL),
-      'base hook' => 'maintenance_page',
+      'variables' => array('content' => NULL),
     ),
     'task_list' => array(
       'variables' => array('items' => NULL, 'active' => NULL),
diff --git a/core/includes/theme.maintenance.inc b/core/includes/theme.maintenance.inc
index b660a10..16cf998 100644
--- a/core/includes/theme.maintenance.inc
+++ b/core/includes/theme.maintenance.inc
@@ -99,14 +99,14 @@ function _theme_load_offline_registry($theme, $base_theme = NULL, $theme_engine
 }
 
 /**
- * Returns HTML for a list of maintenance tasks to perform.
+ * Prepares variables for maintenance task list templates.
  *
- * @param $variables
+ * Default template: task-list.html.twig.
+ *
+ * @param array $variables
  *   An associative array containing:
  *   - items: An associative array of maintenance tasks.
  *   - active: The key for the currently active maintenance task.
- *
- * @ingroup themeable
  */
 function template_preprocess_task_list(&$variables) {
   $t = get_t();
@@ -121,12 +121,12 @@ function template_preprocess_task_list(&$variables) {
     $tasks[$k]['attributes'] = array('class' => array());
     if ($active == $k) {
       $tasks[$k]['attributes']['class'][] = 'active';
-      $tasks[$k]['status'] = '(' . $t('active') . ')';
+      $tasks[$k]['status'] = $t('active');
       $done = FALSE;
     }
     else {
       $tasks[$k]['attributes']['class'][] = $done ? 'done' : '';
-      $tasks[$k]['status'] = $done ? '(' . $t('done') . ')' : '';
+      $tasks[$k]['status'] = $done ? $t('done') : '';
     }
     $tasks[$k]['attributes'] = new Attribute($tasks[$k]['attributes']);
   }
@@ -135,7 +135,7 @@ function template_preprocess_task_list(&$variables) {
 }
 
 /**
- * Preprocess variables for the installation page.
+ * Returns HTML for the installation page.
  *
  * Note: this function is not themeable.
  *
@@ -143,13 +143,13 @@ function template_preprocess_task_list(&$variables) {
  *   An associative array containing:
  *   - content: The page content to show.
  */
-function template_preprocess_install_page(&$variables) {
+function theme_install_page($variables) {
   drupal_add_http_header('Content-Type', 'text/html; charset=utf-8');
-  $variables['theme_hook_original'] = 'maintenance_page';
+  return theme('maintenance_page', $variables);
 }
 
 /**
- * Preprocess variables for the update page.
+ * Returns HTML for the update page.
  *
  * Note: this function is not themeable.
  *
@@ -159,19 +159,19 @@ function template_preprocess_install_page(&$variables) {
  *   - show_messages: Whether to output status and error messages.
  *     FALSE can be useful to postpone the messages to a subsequent page.
  */
-function template_preprocess_update_page(&$variables) {
+function theme_update_page($variables) {
   drupal_add_http_header('Content-Type', 'text/html; charset=utf-8');
-  $variables['theme_hook_original'] = 'maintenance_page';
+  return theme('maintenance_page', $variables);
 }
 
 /**
- * Preprocess variables for a report of the results from an operation run via authorize.php.
+ * Prepares variables for authorize.php operation report templates.
  *
- * @param $variables
+ * Default template: authorize-report.html.twig.
+ *
+ * @param array $variables
  *   An associative array containing:
  *   - messages: An array of result messages.
- *
- * @ingroup themeable
  */
 function template_preprocess_authorize_report(&$variables) {
   $messages = $variables['messages'];
diff --git a/core/modules/system/templates/task-list.html.twig b/core/modules/system/templates/task-list.html.twig
index 94d5595..6e4bce4 100644
--- a/core/modules/system/templates/task-list.html.twig
+++ b/core/modules/system/templates/task-list.html.twig
@@ -4,8 +4,12 @@
  * Default theme implementation for a list of maintenance tasks to perform.
  *
  * Available variables:
- *   - attributes: Information related to the maintenance tasks.
- *   - items: An associative array of maintenance tasks.
+ * - tasks: A list of maintenance tasks to perform. Each item in the list has
+ *   the following variables:
+ *   - attributes: HTML attributes for the maintenance task.
+ *   - item: An associative array of maintenance tasks.
+ *   - status: (optional) Text describing the status of the maintenance task,
+ *     'active' or 'done'.
  *
  * @see template_preprocess()
  *
@@ -17,7 +21,7 @@
 {% for task in tasks %}
   <li class="{{ task.attributes.class }}">
     {{- task.item -}}
-    {%- if task.status %} <span class="element-invisible">{{ task.status }}</span>{% endif -%}
+    {%- if task.status %} <span class="element-invisible">({{ task.status }})</span>{% endif -%}
   </li>
 {% endfor %}
 </ol>
diff --git a/core/update.php b/core/update.php
index e56d085..4947d09 100644
--- a/core/update.php
+++ b/core/update.php
@@ -390,7 +390,8 @@ function update_check_requirements($skip_warnings = FALSE) {
     drupal_set_title('Requirements problem');
     $status_report = theme('status_report', array('requirements' => $requirements));
     $status_report .= 'Check the messages and <a href="' . check_url(drupal_requirements_url($severity)) . '">try again</a>.';
-    print theme('update_page', array('content' => $status_report));
+    drupal_add_http_header('Content-Type', 'text/html; charset=utf-8');
+    print theme('maintenance_page', array('content' => $status_report));
     exit();
   }
 }
@@ -533,6 +534,7 @@ function update_check_requirements($skip_warnings = FALSE) {
     $output->send();
   }
   else {
-    print theme('update_page', array('content' => $output, 'show_messages' => !$progress_page));
+    drupal_add_http_header('Content-Type', 'text/html; charset=utf-8');
+    print theme('maintenance_page', array('content' => $output, 'show_messages' => !$progress_page));
   }
 }