diff --git a/core/modules/system/lib/Drupal/system/Plugin/Block/SystemPoweredByBlock.php b/core/modules/system/lib/Drupal/system/Plugin/Block/SystemPoweredByBlock.php
index 7edd34b..710034a 100644
--- a/core/modules/system/lib/Drupal/system/Plugin/Block/SystemPoweredByBlock.php
+++ b/core/modules/system/lib/Drupal/system/Plugin/Block/SystemPoweredByBlock.php
@@ -26,9 +26,7 @@ class SystemPoweredByBlock extends BlockBase {
    * {@inheritdoc}
    */
   public function build() {
-    return array(
-      '#children' => theme('system_powered_by'),
-    );
+    return array('#theme' => 'system_powered_by');
   }
 
 }
diff --git a/core/modules/system/system.admin.inc b/core/modules/system/system.admin.inc
index 05c67a7..46c0aab 100644
--- a/core/modules/system/system.admin.inc
+++ b/core/modules/system/system.admin.inc
@@ -43,8 +43,11 @@ function system_admin_config_page() {
           unset($item['localized_options']['attributes']['title']);
         }
         $block = $item;
-        $block['content'] = '';
-        $block['content'] .= theme('admin_block_content', array('content' => system_admin_menu_block($item)));
+        $admin_block_content = array(
+          '#theme' => 'admin_block_content',
+          '#content' => system_admin_menu_block($item),
+        );
+        $block['content'] = drupal_render($admin_block_content);
         if (!empty($block['content'])) {
           $block['show'] = TRUE;
         }
@@ -57,7 +60,11 @@ function system_admin_config_page() {
   }
   if ($blocks) {
     ksort($blocks);
-    return theme('admin_page', array('blocks' => $blocks));
+    $admin_page = array(
+      '#theme' => 'admin_page',
+      '#blocks' => $blocks,
+    );
+    return drupal_render($admin_page);
   }
   else {
     return t('You do not have any administrative items.');
@@ -77,8 +84,9 @@ function system_admin_config_page() {
  */
 function system_admin_menu_block_page() {
   $item = menu_get_item();
-  if ($content = system_admin_menu_block($item)) {
-    $output = theme('admin_block_content', array('content' => $content));
+  $admin_block_content = array('#theme' => 'admin_block_content');
+  if ($admin_block_content['#content'] = system_admin_menu_block($item)) {
+    $output = drupal_render($admin_block_content);
   }
   else {
     $output = t('You do not have any administrative items.');
@@ -212,8 +220,14 @@ function system_themes_page() {
   uasort($theme_groups['enabled'], 'system_sort_themes');
   drupal_alter('system_themes_page', $theme_groups);
 
-  $admin_form = drupal_get_form('system_themes_admin_form', $admin_theme_options);
-  return theme('system_themes_page', array('theme_groups' => $theme_groups, 'theme_group_titles' => $theme_group_titles)) . drupal_render($admin_form);
+  $build = array();
+  $build['system_themes_page'] = array(
+    '#theme' => 'system_themes_page',
+    '#theme_groups' => $theme_groups,
+    '#theme_group_titles' => $theme_group_titles,
+  );
+  $build['admin_form'] = drupal_get_form('system_themes_admin_form', $admin_theme_options);
+  return $build;
 }
 
 /**
@@ -462,7 +476,11 @@ function theme_admin_page($variables) {
   $container = array();
 
   foreach ($blocks as $block) {
-    if ($block_output = theme('admin_block', array('block' => $block))) {
+    $admin_block = array(
+      '#theme' => 'admin_block',
+      '#block' => $block,
+    );
+    if ($block_output = drupal_render($admin_block)) {
       if (empty($block['position'])) {
         // perform automatic striping.
         $block['position'] = ++$stripe % 2 ? 'left' : 'right';
@@ -474,8 +492,9 @@ function theme_admin_page($variables) {
     }
   }
 
+  $system_compact_link = array('#theme' => 'system_compact_link');
   $output = '<div class="admin clearfix">';
-  $output .= theme('system_compact_link');
+  $output .= drupal_render($system_compact_link);
 
   foreach ($container as $id => $data) {
     $output .= '<div class="' . $id . ' clearfix">';
@@ -509,13 +528,21 @@ function theme_system_admin_index($variables) {
 
     // Output links.
     if (count($items)) {
+      $admin_block_content = array(
+        '#theme' => 'admin_block_content',
+        '#content' => $items,
+      );
       $block = array();
       $block['title'] = $module;
-      $block['content'] = theme('admin_block_content', array('content' => $items));
+      $block['content'] = drupal_render($admin_block_content);
       $block['description'] = t($description);
       $block['show'] = TRUE;
 
-      if ($block_output = theme('admin_block', array('block' => $block))) {
+      $admin_block = array(
+        '#theme' => 'admin_block',
+        '#block' => $block,
+      );
+      if ($block_output = drupal_render($admin_block)) {
         if (!isset($block['position'])) {
           // Perform automatic striping.
           $block['position'] = $position;
@@ -526,8 +553,9 @@ function theme_system_admin_index($variables) {
     }
   }
 
+  $system_compact_link = array('#theme' => 'system_compact_link');
   $output = '<div class="admin clearfix">';
-  $output .= theme('system_compact_link');
+  $output .= drupal_render($system_compact_link);
   foreach ($container as $id => $data) {
     $output .= '<div class="' . $id . ' clearfix">';
     $output .= $data;
@@ -675,7 +703,12 @@ function theme_system_modules_details($variables) {
     $rows[] = $row;
   }
 
-  return theme('table', array('header' => $form['#header'], 'rows' => $rows));
+  $table = array(
+    '#theme' => 'table',
+    '#header' => $form['#header'],
+    '#rows' => $rows,
+  );
+  return drupal_render($table);
 }
 
 /**
@@ -734,7 +767,13 @@ function theme_system_modules_uninstall($variables) {
     );
   }
 
-  $output  = theme('table', array('header' => $header, 'rows' => $rows, 'empty' => t('No modules are available to uninstall.')));
+  $table = array(
+    '#theme' => 'table',
+    '#header' => $header,
+    '#rows' => $rows,
+    '#empty' => t('No modules are available to uninstall.'),
+  );
+  $output  = drupal_render($table);
   $output .= drupal_render_children($form);
 
   return $output;
@@ -746,6 +785,7 @@ function theme_system_modules_uninstall($variables) {
  * @param $variables
  *   An associative array containing:
  *   - theme_groups: An associative array containing groups of themes.
+ *   - theme_group_titles: An associative array containing titles of themes.
  *
  * @ingroup themeable
  */
@@ -853,7 +893,12 @@ function system_date_format_language_overview_page() {
     $rows[] = $row;
   }
 
-  return theme('table', array('header' => $header, 'rows' => $rows));
+  $table = array(
+    '#theme' => 'table',
+    '#header' => $header,
+    '#rows' => $rows,
+  );
+  return drupal_render($table);
 }
 
 /**
@@ -961,8 +1006,13 @@ function theme_system_date_format_localize_form($variables) {
     $rows[] = $row;
   }
 
+  $table = array(
+    '#theme' => 'table',
+    '#header' => $header,
+    '#rows' => $rows,
+  );
   $output = drupal_render($form['language']);
-  $output .= theme('table', array('header' => $header, 'rows' => $rows));
+  $output .= drupal_render($table);
   $output .= drupal_render_children($form);
 
   return $output;
diff --git a/core/modules/system/system.api.php b/core/modules/system/system.api.php
index 8ef7556..94e595e 100644
--- a/core/modules/system/system.api.php
+++ b/core/modules/system/system.api.php
@@ -410,7 +410,8 @@ function hook_css_alter(&$css) {
  */
 function hook_ajax_render_alter($commands) {
   // Inject any new status messages into the content area.
-  $commands[] = ajax_command_prepend('#block-system-main .content', theme('status_messages'));
+  $status_messages = array('#theme' => 'status_messages');
+  $commands[] = ajax_command_prepend('#block-system-main .content', drupal_render($status_messages));
 }
 
 /**
diff --git a/core/modules/system/system.install b/core/modules/system/system.install
index 4d882dd..a07c976 100644
--- a/core/modules/system/system.install
+++ b/core/modules/system/system.install
@@ -131,7 +131,11 @@ function system_requirements($phase) {
       '@system_requirements' => 'http://drupal.org/requirements',
     ));
 
-    $description .= theme('item_list', array('items' => $missing_extensions));
+    $item_list = array(
+      '#theme' => 'item_list',
+      '#items' => $missing_extensions,
+    );
+    $description .= drupal_render($item_list);
 
     $requirements['php_extensions']['value'] = t('Disabled');
     $requirements['php_extensions']['severity'] = REQUIREMENT_ERROR;
@@ -246,7 +250,11 @@ function system_requirements($phase) {
         $description = $conf_errors[0];
       }
       else {
-        $description = theme('item_list', array('items' => $conf_errors));
+        $item_list = array(
+          '#theme' => 'item_list',
+          '#items' => $conf_errors,
+        );
+        $description = drupal_render($item_list);
       }
       $requirements['settings.php'] = array(
         'value' => t('Not protected'),
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index 1935f5c..12e6cbe 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -140,7 +140,10 @@ function system_help($path, $arg) {
 function system_theme() {
   return array_merge(drupal_common_theme(), array(
     'system_themes_page' => array(
-      'variables' => array('theme_groups' => NULL),
+      'variables' => array(
+        'theme_groups' => NULL,
+        'theme_group_titles' => NULL,
+      ),
       'file' => 'system.admin.inc',
     ),
     'system_config_form' => array(
@@ -3342,7 +3345,12 @@ function theme_exposed_filters($variables) {
     foreach (element_children($form['current']) as $key) {
       $items[] = $form['current'][$key];
     }
-    $output .= theme('item_list', array('items' => $items, 'attributes' => array('class' => array('clearfix', 'current-filters'))));
+    $item_list = array(
+      '#theme' => 'item_list',
+      '#items' => $items,
+      '#attributes' => array('class' => array('clearfix', 'current-filters')),
+    );
+    $output .= drupal_render($item_list);
   }
 
   $output .= drupal_render_children($form);
diff --git a/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/EventSubscriber/ThemeTestSubscriber.php b/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/EventSubscriber/ThemeTestSubscriber.php
index 56cbdaf..6e28a15 100644
--- a/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/EventSubscriber/ThemeTestSubscriber.php
+++ b/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/EventSubscriber/ThemeTestSubscriber.php
@@ -34,7 +34,12 @@ public function onRequest(GetResponseEvent $event) {
       // theme_test_request_listener_page_callback() to test that even when the
       // theme system is initialized this early, it is still capable of
       // returning output and theming the page as a whole.
-      $GLOBALS['theme_test_output'] = theme('more_link', array('url' => 'user', 'title' => 'Themed output generated in a KernelEvents::REQUEST listener'));
+      $more_link = array(
+        '#theme' => 'more_link',
+        '#url' => 'user',
+        '#title' => 'Themed output generated in a KernelEvents::REQUEST listener',
+      );
+      $GLOBALS['theme_test_output'] = drupal_render($more_link);
     }
     if (strpos($current_path, 'user/autocomplete') === 0) {
       // Register a fake registry loading callback. If it gets called by
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.module b/core/modules/system/tests/modules/theme_test/theme_test.module
index 51cb6d3..13a0243 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.module
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -9,9 +9,11 @@ function theme_test_theme($existing, $type, $theme, $path) {
     'variables' => array('foo' => ''),
   );
   $items['theme_test_template_test'] = array(
+    'variables' => array(),
     'template' => 'theme_test.template_test',
   );
   $items['theme_test_template_test_2'] = array(
+    'variables' => array(),
     'template' => 'theme_test.template_test',
   );
   $items['theme_test_foo'] = array(
@@ -100,7 +102,8 @@ function theme_test_request_listener_page_callback() {
  * Menu callback for testing template overridding based on filename.
  */
 function theme_test_template_test_page_callback() {
-  return theme('theme_test_template_test');
+  $theme_test_template_test = array('#theme' => 'theme_test_template_test');
+  return drupal_render($theme_test_template_test);
 }
 
 /**
@@ -146,7 +149,8 @@ function _theme_test_alter() {
  * Page callback, calls a theme hook suggestion.
  */
 function _theme_test_suggestion() {
-  return theme(array('theme_test__suggestion', 'theme_test'), array());
+  $theme_test = array('#theme' => array('theme_test__suggestion', 'theme_test'));
+  return drupal_render($theme_test);
 }
 
 /**
diff --git a/core/modules/system/tests/modules/twig_theme_test/lib/Drupal/twig_theme_test/TwigThemeTestController.php b/core/modules/system/tests/modules/twig_theme_test/lib/Drupal/twig_theme_test/TwigThemeTestController.php
index 6eb318a..e4ba607 100644
--- a/core/modules/system/tests/modules/twig_theme_test/lib/Drupal/twig_theme_test/TwigThemeTestController.php
+++ b/core/modules/system/tests/modules/twig_theme_test/lib/Drupal/twig_theme_test/TwigThemeTestController.php
@@ -26,7 +26,8 @@ public static function create(ContainerInterface $container) {
    * Menu callback for testing PHP variables in a Twig template.
    */
   public function phpVariablesRender() {
-    return theme('twig_theme_test_php_variables');
+    $twig_theme_test_php_variables = array('#theme' => 'twig_theme_test_php_variables');
+    return drupal_render($twig_theme_test_php_variables);
   }
 
   /**
diff --git a/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.module b/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.module
index 934d1b0..cc74184 100644
--- a/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.module
+++ b/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.module
@@ -5,6 +5,7 @@
  */
 function twig_theme_test_theme($existing, $type, $theme, $path) {
   $items['twig_theme_test_php_variables'] = array(
+    'variables' => array(),
     'template' => 'twig_theme_test.php_variables',
   );
   $items['twig_theme_test_trans'] = array(