diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index e8cff8b..b9080b7 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 & 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.
   $variables['directory'] = \Drupal::theme()->getActiveTheme()->getPath();
@@ -1259,6 +1260,7 @@ function template_preprocess_html(&$variables) {
     $system_path = \Drupal::service('path.current')->getPath();
     $variables['root_path'] = explode('/', $system_path)[1];
   }
+  $variables['base_path']         = base_path();
 
   $site_config = \Drupal::config('system.site');
   // Construct page title.
diff --git a/core/modules/system/templates/html.html.twig b/core/modules/system/templates/html.html.twig
index 39702c2..43ce0ce 100644
--- a/core/modules/system/templates/html.html.twig
+++ b/core/modules/system/templates/html.html.twig
@@ -5,6 +5,8 @@
  *
  * Variables:
  * - logged_in: A flag indicating if user is logged in.
+ * - base_path: The base URL path of the Drupal installation. Will usually be
+ *   "/" unless you have installed Drupal in a sub-directory.
  * - root_path: The root path of the current page (e.g., node, admin, user).
  * - node_type: The content type for the current node, if the page is a node.
  * - head_title: List of text elements that make up the head_title variable.
diff --git a/core/themes/classy/templates/layout/html.html.twig b/core/themes/classy/templates/layout/html.html.twig
index b787ed1..2ba07b7 100644
--- a/core/themes/classy/templates/layout/html.html.twig
+++ b/core/themes/classy/templates/layout/html.html.twig
@@ -5,6 +5,8 @@
  *
  * Variables:
  * - logged_in: A flag indicating if user is logged in.
+ * - base_path: The base URL path of the Drupal installation. Will usually be
+ *   "/" unless you have installed Drupal in a sub-directory.
  * - root_path: The root path of the current page (e.g., node, admin, user).
  * - node_type: The content type for the current node, if the page is a node.
  * - head_title: List of text elements that make up the head_title variable.