diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index e8cff8b..913ad30 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -1204,8 +1204,9 @@ function _template_preprocess_default_variables() {
     'logged_in' => FALSE,
   );
 
-  // Give modules a chance to alter the default template variables.
+  // Give modules and themes a chance to alter the default template variables.
   \Drupal::moduleHandler()->alter('template_preprocess_default_variables', $variables);
+  \Drupal::themeHandler()->alter('template_preprocess_default_variables', $variables);
 
   // Tell all templates where they are located.
   $variables['directory'] = \Drupal::theme()->getActiveTheme()->getPath();