diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index ff4f678..37c6022 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -350,9 +350,9 @@ protected function build() {
     // Finally, hooks provided by the theme itself.
     $this->processExtension($cache, $this->theme->getName(), 'theme', $this->theme->getName(), $this->theme->getPath());
 
-    // Let modules alter the registry.
+    // Let themes and modules alter the registry.
     $this->moduleHandler->alter('theme_registry', $cache);
-    // @todo Do we want to allow themes to take part?
+    \Drupal::theme()->alter('theme_registry', $cache);
 
     // @todo Implement more reduction of the theme registry entry.
     // Optimize the registry to not have empty arrays for functions.