diff -u b/core/lib/Drupal/Core/Template/TwigExtension.php b/core/lib/Drupal/Core/Template/TwigExtension.php
--- b/core/lib/Drupal/Core/Template/TwigExtension.php
+++ b/core/lib/Drupal/Core/Template/TwigExtension.php
@@ -80,7 +80,7 @@
       new \Twig_SimpleFunction('url_from_path', array($this, 'getUrlFromPath'), array('is_safe_callback' => array($this, 'isUrlGenerationSafe'))),
       new \Twig_SimpleFunction('link', array($this, 'getLink')),
       new \Twig_SimpleFunction('file_url', 'file_create_url'),
-      new \Twig_SimpleFunction('active_theme', 'activeTheme'),
+      new \Twig_SimpleFunction('active_theme', array($this, 'activeTheme')),
     );
   }
 
only in patch2:
unchanged:
--- a/core/modules/system/src/Tests/Theme/EngineTwigTest.php
+++ b/core/modules/system/src/Tests/Theme/EngineTwigTest.php
@@ -111,4 +111,12 @@ public function testTwigFileUrls() {
     $this->assertRaw('<div>file_url: ' . $filepath . '</div>');
   }
 
+  /**
+   * Tests the active_theme() function.
+   */
+  public function testActiveTheme() {
+    $this->drupalGet('/twig-theme-test/active_theme');
+    $this->assertRaw('<div>active theme: classy</div>');
+  }
+
 }
only in patch2:
unchanged:
--- a/core/modules/system/tests/modules/twig_theme_test/src/TwigThemeTestController.php
+++ b/core/modules/system/tests/modules/twig_theme_test/src/TwigThemeTestController.php
@@ -75,4 +75,11 @@ public function registryLoaderRender() {
     return array('#theme' => 'twig_registry_loader_test');
   }
 
+  /**
+   * Menu callback for testing the active theme function.
+   */
+  public function activeThemeRender() {
+    return array('#theme' => 'twig_theme_test_active_theme');
+  }
+
 }
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/system/tests/modules/twig_theme_test/templates/twig_theme_test.active_theme.html.twig
@@ -0,0 +1 @@
+<div>active theme: {{ active_theme() }}</div>
only in patch2:
unchanged:
--- 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
@@ -50,6 +50,10 @@ function twig_theme_test_theme($existing, $type, $theme, $path) {
     'variables' => array(),
     'template' => 'twig_theme_test.file_url',
   );
+  $items['twig_theme_test_active_theme'] = array(
+    'variables' => array(),
+    'template' => 'twig_theme_test.active_theme',
+  );
   return $items;
 }
 
only in patch2:
unchanged:
--- a/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.routing.yml
+++ b/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.routing.yml
@@ -46,3 +46,10 @@ twig_theme_test_registry_loader:
     _controller: '\Drupal\twig_theme_test\TwigThemeTestController::registryLoaderRender'
   requirements:
     _access: 'TRUE'
+
+twig_theme_test_active_theme:
+  path: '/twig-theme-test/active_theme'
+  defaults:
+    _controller: '\Drupal\twig_theme_test\TwigThemeTestController::activeThemeRender'
+  requirements:
+    _access: 'TRUE'