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
@@ -98,8 +98,8 @@
       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('active_theme', [$this, 'activeTheme']),
+      new \Twig_SimpleFunction('attach_library', [$this, 'attachLibrary']),
+      new \Twig_SimpleFunction('active_theme', [$this, 'getActiveTheme']),
     ];
   }
 
@@ -266,7 +266,7 @@
    * @return string
    *   The name of the active theme.
    */
-  public function activeTheme() {
+  public function getActiveTheme() {
     return $this->themeManager->getActiveTheme()->getName();
   }