diff --git a/core/core.services.yml b/core/core.services.yml
index 58db84c..97e408b 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -1322,7 +1322,7 @@ services:
     tags:
       - { name: twig.extension, priority: 100 }
     calls:
-      - [setGenerators, ['@url_generator']]
+      - [setGenerators, ['@url_generator', '@theme.manager']]
   # @todo Figure out what to do about debugging functions.
   # @see https://www.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 1902113..88c6add 100644
--- a/core/lib/Drupal/Core/Template/TwigExtension.php
+++ b/core/lib/Drupal/Core/Template/TwigExtension.php
@@ -15,6 +15,7 @@
 use Drupal\Component\Utility\SafeMarkup;
 use Drupal\Core\Render\RendererInterface;
 use Drupal\Core\Routing\UrlGeneratorInterface;
+use Drupal\Core\Theme\ThemeManagerInterface;
 use Drupal\Core\Url;
 
 /**
@@ -41,6 +42,13 @@ class TwigExtension extends \Twig_Extension {
   protected $renderer;
 
   /**
+   * The theme manager.
+   *
+   * @var \Drupal\Core\Theme\ThemeManagerInterface
+   */
+  protected $themeManager;
+
+  /**
    * Constructs \Drupal\Core\Template\TwigExtension.
    *
    * @param \Drupal\Core\Render\RendererInterface $renderer
@@ -58,8 +66,9 @@ public function __construct(RendererInterface $renderer) {
    *
    * @return $this
    */
-  public function setGenerators(UrlGeneratorInterface $url_generator) {
+  public function setGenerators(UrlGeneratorInterface $url_generator, ThemeManagerInterface $theme_manager) {
     $this->urlGenerator = $url_generator;
+    $this->themeManager = $theme_manager;
     return $this;
   }
 
@@ -67,7 +76,7 @@ public function setGenerators(UrlGeneratorInterface $url_generator) {
    * {@inheritdoc}
    */
   public function getFunctions() {
-    return array(
+    return [
       // This function will receive a renderable array, if an array is detected.
       new \Twig_SimpleFunction('render_var', array($this, 'renderVar')),
       // The url and path function are defined in close parallel to those found
@@ -77,8 +86,9 @@ public function getFunctions() {
       new \Twig_SimpleFunction('url_from_path', array($this, 'getUrlFromPath'), array('is_safe_callback' => array($this, 'isUrlGenerationSafe'))),
       new \Twig_SimpleFunction('link', array($this, 'getLink')),
       new \Twig_SimpleFunction('file_url', 'file_create_url'),
-      new \Twig_SimpleFunction('attach_library', array($this, 'attachLibrary'))
-    );
+      new \Twig_SimpleFunction('attach_library', [$this, 'attachLibrary']),
+      new \Twig_SimpleFunction('active_theme', [$this, 'getActiveTheme']),
+    ];
   }
 
   /**
@@ -247,6 +257,16 @@ public function getLink($text, $url, array $attributes = []) {
   }
 
   /**
+   * Gets the name of the active theme.
+   *
+   * @return string
+   *   The name of the active theme.
+   */
+  public function getActiveTheme() {
+    return $this->themeManager->getActiveTheme()->getName();
+  }
+
+  /**
    * Determines at compile time whether the generated URL will be safe.
    *
    * Saves the unneeded automatic escaping for performance reasons.
diff --git a/core/tests/Drupal/Tests/Core/Template/TwigExtensionTest.php b/core/tests/Drupal/Tests/Core/Template/TwigExtensionTest.php
index 3866965..06d6dc3 100644
--- a/core/tests/Drupal/Tests/Core/Template/TwigExtensionTest.php
+++ b/core/tests/Drupal/Tests/Core/Template/TwigExtensionTest.php
@@ -32,7 +32,7 @@ public function testEscaping($template, $expected) {
       'autoescape' => TRUE,
       'optimizations' => 0
     ));
-    $twig->addExtension((new TwigExtension($renderer))->setGenerators($this->getMock('Drupal\Core\Routing\UrlGeneratorInterface')));
+    $twig->addExtension((new TwigExtension($renderer))->setGenerators($this->getMock('Drupal\Core\Routing\UrlGeneratorInterface'), $this->getMock('Drupal\Core\Theme\ThemeManagerInterface')));
 
     $nodes = $twig->parse($twig->tokenize($template));
 
@@ -72,4 +72,32 @@ public function providerTestEscaping() {
     );
   }
 
+  /**
+   * Tests the active_theme function.
+   */
+  public function testActiveTheme() {
+    $renderer = $this->getMock('\Drupal\Core\Render\RendererInterface');
+    $extension = new TwigExtension($renderer);
+    $url_generator = $this->getMock('\Drupal\Core\Routing\UrlGeneratorInterface');
+    $theme_manager = $this->getMock('\Drupal\Core\Theme\ThemeManagerInterface');
+    $active_theme = $this->getMockBuilder('\Drupal\Core\Theme\ActiveTheme')
+      ->disableOriginalConstructor()
+      ->getMock();
+    $active_theme
+      ->expects($this->once())
+      ->method('getName')
+      ->willReturn('test_theme');
+    $theme_manager
+      ->expects($this->once())
+      ->method('getActiveTheme')
+      ->willReturn($active_theme);
+    $extension->setGenerators($url_generator, $theme_manager);
+
+    $loader = new \Twig_Loader_String();
+    $twig = new \Twig_Environment($loader);
+    $twig->addExtension($extension);
+    $result = $twig->render('{{ active_theme() }}');
+    $this->assertEquals('test_theme', $result);
+  }
+
 }