diff --git a/core/core.services.yml b/core/core.services.yml
index a7a2b3b..6c6f229 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -1219,8 +1219,7 @@ services:
     tags:
       - { name: twig.extension, priority: 100 }
     calls:
-      - [setGenerators, ['@url_generator']]
-      - [setLinkGenerator, ['@link_generator']]
+      - [setDependencies, ['@url_generator', '@link_generator', '@theme.manager']]
   # @todo Figure out what to do about debugging functions.
   # @see http://drupal.org/node/1804998
   twig.extension.debug:
diff --git a/core/lib/Drupal/Core/Template/TwigExtension.php b/core/lib/Drupal/Core/Template/TwigExtension.php
index 551e3cb..0934c68 100644
--- a/core/lib/Drupal/Core/Template/TwigExtension.php
+++ b/core/lib/Drupal/Core/Template/TwigExtension.php
@@ -15,6 +15,7 @@
 use Drupal\Core\Routing\UrlGeneratorInterface;
 use Drupal\Core\Url;
 use Drupal\Core\Utility\LinkGeneratorInterface;
+use Drupal\Core\Theme\ThemeManagerInterface;
 
 /**
  * A class providing Drupal Twig extensions.
@@ -40,26 +41,28 @@ class TwigExtension extends \Twig_Extension {
   protected $linkGenerator;
 
   /**
-   * Constructs \Drupal\Core\Template\TwigExtension.
+   * The theme manager.
    *
-   * @param \Drupal\Core\Routing\UrlGeneratorInterface $url_generator
-   *   The URL generator.
+   * @var \Drupal\Core\Theme\ThemeManagerInterface;
    */
-  public function setGenerators(UrlGeneratorInterface $url_generator) {
-    $this->urlGenerator = $url_generator;
-    return $this;
-  }
+  protected $themeManager;
 
   /**
-   * Sets the link generator.
+   * Constructs \Drupal\Core\Template\TwigExtension.
    *
+   * @param \Drupal\Core\Routing\UrlGeneratorInterface $url_generator
+   *   The URL generator.
    * @param \Drupal\Core\Utility\LinkGeneratorInterface $link_generator
    *   The link generator.
+   * @param \Drupal\Core\Theme\ThemeManagerInterface $theme_manager
+   *   The theme manager.
    *
    * @return $this
    */
-  public function setLinkGenerator(LinkGeneratorInterface $link_generator) {
+  public function setDependencies(UrlGeneratorInterface $url_generator, LinkGeneratorInterface $link_generator, ThemeManagerInterface $theme_manager) {
+    $this->urlGenerator = $url_generator;
     $this->linkGenerator = $link_generator;
+    $this->themeManager = $theme_manager;
     return $this;
   }
 
@@ -77,6 +80,7 @@ public function getFunctions() {
       new \Twig_SimpleFunction('url_from_path', array($this, 'getUrlFromPath'), array('is_safe_callback' => array($this, 'isUrlGenerationSafe'))),
       new \Twig_SimpleFunction('link', array($this, 'getLink')),
       new \Twig_SimpleFunction('file_url', 'file_create_url'),
+      new \Twig_SimpleFunction('active_theme', array($this, 'activeTheme')),
     );
   }
 
@@ -219,6 +223,16 @@ public function getLink($text, $url) {
   }
 
   /**
+   * Gets the name of the active theme.
+   *
+   * @return string
+   *   The name of the active theme.
+   */
+  public function activeTheme() {
+    return $this->themeManager->getActiveTheme()->getName();
+  }
+
+  /**
    * Determines at compile time whether the generated URL will be safe.
    *
    * Saves the unneeded automatic escaping for performance reasons.
diff --git a/core/modules/system/src/Tests/Theme/EngineTwigTest.php b/core/modules/system/src/Tests/Theme/EngineTwigTest.php
index 750ae2e..9989e60 100644
--- a/core/modules/system/src/Tests/Theme/EngineTwigTest.php
+++ b/core/modules/system/src/Tests/Theme/EngineTwigTest.php
@@ -111,4 +111,13 @@ 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');
+    $theme = \Drupal::service('theme.manager')->getActiveTheme()->getName();
+    $this->assertRaw('<div>active theme: ' . $theme . '</div>');
+  }
+
 }
diff --git a/core/modules/system/tests/modules/twig_theme_test/src/TwigThemeTestController.php b/core/modules/system/tests/modules/twig_theme_test/src/TwigThemeTestController.php
index ff49bd6..15a8bbb 100644
--- 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');
+  }
+
 }
diff --git a/core/modules/system/tests/modules/twig_theme_test/templates/twig_theme_test.active_theme.html.twig b/core/modules/system/tests/modules/twig_theme_test/templates/twig_theme_test.active_theme.html.twig
new file mode 100644
index 0000000..fdf29fe
--- /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>
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 7f6fe35..8c7873e 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
@@ -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;
 }
 
diff --git 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
index 1001b47..c0b8116 100644
--- 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'
diff --git a/core/tests/Drupal/Tests/Core/Template/TwigExtensionTest.php b/core/tests/Drupal/Tests/Core/Template/TwigExtensionTest.php
index 31352a6..035fadb 100644
--- a/core/tests/Drupal/Tests/Core/Template/TwigExtensionTest.php
+++ b/core/tests/Drupal/Tests/Core/Template/TwigExtensionTest.php
@@ -31,7 +31,13 @@ public function testEscaping($template, $expected) {
       'autoescape' => TRUE,
       'optimizations' => 0
     ));
-    $twig->addExtension((new TwigExtension())->setGenerators($this->getMock('Drupal\Core\Routing\UrlGeneratorInterface')));
+    $twig->addExtension((new TwigExtension())
+      ->setDependencies(
+        $this->getMock('Drupal\Core\Routing\UrlGeneratorInterface'),
+        $this->getMock('Drupal\Core\Utility\LinkGeneratorInterface'),
+        $this->getMock('Drupal\Core\Theme\ThemeManagerInterface')
+      )
+    );
 
     $nodes = $twig->parse($twig->tokenize($template));