diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index f98aadd..a250f56b 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -8,6 +8,7 @@
  * customized by user themes.
  */
 
+use Drupal;
 use Drupal\Component\Utility\String;
 use Drupal\Component\Utility\Url;
 use Drupal\Core\Config\Config;
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
index 3917ffa..e22ec6e 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
@@ -2888,7 +2888,7 @@ protected function assertNoTitle($title, $message = '', $group = 'Other') {
    *   TRUE on pass, FALSE on fail.
    */
   protected function assertThemeOutput($callback, array $variables = array(), $expected, $message = '', $group = 'Other') {
-    $output = theme($callback, $variables);
+    $output = drupal_render(array('#theme' => $callback, $variables);
     $this->verbose('Variables:' . '<pre>' .  check_plain(var_export($variables, TRUE)) . '</pre>'
       . '<hr />' . 'Result:' . '<pre>' .  check_plain(var_export($output, TRUE)) . '</pre>'
       . '<hr />' . 'Expected:' . '<pre>' .  check_plain(var_export($expected, TRUE)) . '</pre>'
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/TableTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/TableTest.php
index 1a9d3d1..b4c843a 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/TableTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/TableTest.php
@@ -27,7 +27,7 @@ public static function getInfo() {
   function testThemeTableStickyHeaders() {
     $header = array('one', 'two', 'three');
     $rows = array(array(1,2,3), array(4,5,6), array(7,8,9));
-    $this->content = theme('table', array('header' => $header, 'rows' => $rows, 'sticky' => TRUE));
+    $this->content = drupal_render(array('#theme' => 'table', '#header' => $header, '#rows' => $rows, '#sticky' => TRUE));
     $js = _drupal_add_js();
     $this->assertTrue(isset($js['core/misc/tableheader.js']), 'tableheader.js was included when $sticky = TRUE.');
     $this->assertRaw('sticky-enabled',  'Table has a class of sticky-enabled when $sticky = TRUE.');
@@ -43,7 +43,7 @@ function testThemeTableNoStickyHeaders() {
     $attributes = array();
     $caption = NULL;
     $colgroups = array();
-    $this->content = theme('table', array('header' => $header, 'rows' => $rows, 'attributes' => $attributes, 'caption' => $caption, 'colgroups' => $colgroups, 'sticky' => FALSE));
+    $this->content = drupal_render(array('#theme' => 'table', '#header' => $header, '#rows' => $rows, '#attributes' => $attributes, '#caption' => $caption, '#colgroups' => $colgroups, '#sticky' => FALSE));
     $js = _drupal_add_js();
     $this->assertFalse(isset($js['core/misc/tableheader.js']), 'tableheader.js was not included because $sticky = FALSE.');
     $this->assertNoRaw('sticky-enabled',  'Table does not have a class of sticky-enabled because $sticky = FALSE.');
@@ -62,7 +62,7 @@ function testThemeTableWithEmptyMessage() {
         'colspan' => 2,
       ),
     );
-    $this->content = theme('table', array('header' => $header, 'rows' => array(), 'empty' => t('No strings available.')));
+    $this->content = drupal_render(array('#theme' => 'table', '#header' => $header, '#rows' => array(), '#empty' => t('No strings available.')));
     $this->assertRaw('<tr class="odd"><td colspan="3" class="empty message">No strings available.</td>', 'Correct colspan was set on empty message.');
     $this->assertRaw('<thead><tr><th>Header 1</th>', 'Table header was printed.');
   }
@@ -77,7 +77,7 @@ function testThemeTableWithNoStriping() {
         'no_striping' => TRUE,
       ),
     );
-    $this->content = theme('table', array('rows' => $rows));
+      $this->content = drupal_render(array('#theme' => 'table', '#rows' => $rows));
     $this->assertNoRaw('class="odd"', 'Odd/even classes were not added because $no_striping = TRUE.');
     $this->assertNoRaw('no_striping', 'No invalid no_striping HTML attribute was printed.');
   }
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
index 492dfd6..4e00b40 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
@@ -41,7 +41,7 @@ function setUp() {
    * Render arrays that use a render element and templates (and hence call
    * template_preprocess()) must ensure the attributes at different occasions
    * are all merged correctly:
-   *   - $variables['attributes'] as passed in to theme()
+   *   - $variables['attributes'] as passed in to drupal_render()
    *   - the render element's #attributes
    *   - any attributes set in the template's preprocessing function
    */
@@ -58,21 +58,21 @@ function testAttributeMerging() {
   }
 
   /**
-   * Test that theme() returns expected data types.
+   * Test that drupal_render() returns expected data types.
    */
   function testThemeDataTypes() {
-    // theme_test_false is an implemented theme hook so theme() should return a
+    // theme_test_false is an implemented theme hook so drupal_render() should return a
     // string, even though the theme function itself can return anything.
     $foos = array('null' => NULL, 'false' => FALSE, 'integer' => 1, 'string' => 'foo');
     foreach ($foos as $type => $example) {
-      $output = theme('theme_test_foo', array('foo' => $example));
+      $output = drupal_render(array('#theme' => 'theme_test_foo', '#foo' => $example));
       $this->assertTrue(is_string($output), format_string('theme() returns a string for data type !type.', array('!type' => $type)));
     }
 
-    // suggestionnotimplemented is not an implemented theme hook so theme()
+    // suggestionnotimplemented is not an implemented theme hook so drupal_render()
     // should return FALSE instead of a string.
-    $output = theme(array('suggestionnotimplemented'));
-    $this->assertIdentical($output, FALSE, 'theme() returns FALSE when a hook suggestion is not implemented.');
+    $output = drupal_render(array('#theme' => 'suggestionnotimplemented'));
+    $this->assertIdentical($output, FALSE, 'drupal_render() returns FALSE when a hook suggestion is not implemented.');
   }
 
   /**
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/TwigDebugMarkupTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/TwigDebugMarkupTest.php
index 3a26efe..49ce8d2 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/TwigDebugMarkupTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/TwigDebugMarkupTest.php
@@ -34,7 +34,7 @@ public static function getInfo() {
    */
   function testTwigDebugMarkup() {
     $extension = twig_extension();
-    theme_enable(array('test_theme'));
+    \Drupal::service('theme_handler')->enable((array('test_theme'));
     \Drupal::config('system.theme')->set('default', 'test_theme')->save();
     // Enable debug, rebuild the service container, and clear all caches.
     $this->settingsSet('twig_debug', TRUE);
@@ -48,9 +48,9 @@ function testTwigDebugMarkup() {
 
     // Create a node and test different features of the debug markup.
     $node = $this->drupalCreateNode();
-    $output = theme('node', node_view($node));
+    $output = drupal_render(array('#theme' => 'node', 'node' => node_view($node));
     $this->assertTrue(strpos($output, '<!-- THEME DEBUG -->') !== FALSE, 'Twig debug markup found in theme output when debug is enabled.');
-    $this->assertTrue(strpos($output, "CALL: theme('node')") !== FALSE, 'Theme call information found.');
+    $this->assertTrue(strpos($output, "CALL: drupal_render(array('#theme' =>'node'))") !== FALSE, 'Theme call information found.');
     $this->assertTrue(strpos($output, 'x node--1' . $extension) !== FALSE, 'Node ID specific template shown as current template.');
     $this->assertTrue(strpos($output, '* node' . $extension) !== FALSE, 'Base template file found.');
     $template_filename = $templates['node__1']['path'] . '/' . $templates['node__1']['template'] . $extension;
@@ -59,7 +59,7 @@ function testTwigDebugMarkup() {
     // Create another node and make sure the template suggestions shown in the
     // debug markup are correct.
     $node2 = $this->drupalCreateNode();
-    $output = theme('node', node_view($node2));
+    $output = drupal_render(array('#theme' => 'node', 'node' => node_view($node2));
     $this->assertTrue(strpos($output, '* node--2' . $extension) !== FALSE, 'Node ID specific template suggestion found.');
     $this->assertTrue(strpos($output, 'x node' . $extension) !== FALSE, 'Base template file shown as current template.');
 
@@ -68,7 +68,7 @@ function testTwigDebugMarkup() {
     $this->rebuildContainer();
     $this->resetAll();
 
-    $output = theme('node', node_view($node));
+    $output = drupal_render(array('#theme' => 'node', 'node' => node_view($node));
     $this->assertFalse(strpos($output, '<!-- THEME DEBUG -->') !== FALSE, 'Twig debug markup not found in theme output when debug is disabled.');
   }
 
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 2e5417d..81dd267 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
@@ -42,7 +42,7 @@ 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'));
+      $GLOBALS['theme_test_output'] = drupal_render(array('#theme' => 'more_link', 'url' => 'user', 'title' => 'Themed output generated in a KernelEvents::REQUEST listener'));
     }
   }
 
diff --git a/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/ThemeTestController.php b/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/ThemeTestController.php
index 29bfa4e..75c9c34 100644
--- a/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/ThemeTestController.php
+++ b/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/ThemeTestController.php
@@ -55,7 +55,7 @@ public function testInfoStylesheets() {
    *   A render array containing a theme override.
    */
   public function testTemplate() {
-    return theme('theme_test_template_test');
+    return drupal_render(array('#theme' => 'theme_test_template_test'));
   }
 
   /**
@@ -65,7 +65,7 @@ public function testTemplate() {
    *   An HTML string containing the themed output.
    */
   public function testSuggestion() {
-    return theme(array('theme_test__suggestion', 'theme_test'), array());
+    return drupal_render(array('#theme' => 'theme_test__suggestion', 'theme_test' => array()));
   }
 
 /**
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 b46f948..ca25a8b 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
@@ -16,7 +16,7 @@ class TwigThemeTestController {
    * Menu callback for testing PHP variables in a Twig template.
    */
   public function phpVariablesRender() {
-    return theme('twig_theme_test_php_variables');
+    return drupal_render(array('#theme' => 'twig_theme_test_php_variables'));
   }
 
   /**