diff --git a/core/lib/Drupal/Core/Extension/ThemeHandler.php b/core/lib/Drupal/Core/Extension/ThemeHandler.php
index 227851b..9398072 100644
--- a/core/lib/Drupal/Core/Extension/ThemeHandler.php
+++ b/core/lib/Drupal/Core/Extension/ThemeHandler.php
@@ -391,6 +391,18 @@ public function getBaseThemes(array $themes, $theme) {
     return $this->doGetBaseThemes($themes, $theme);
   }
 
+  /**
+   * Finds the base themes for the specific theme.
+   *
+   * @param array $themes
+   *   An array of available themes.
+   * @param string $theme
+   *   The name of the theme whose base we are looking for.
+   * @param array $used_themes
+   *   (optional) A recursion parameter preventing endless loops. Defaults to
+   *   an empty array.
+   * @return array
+   */
   protected function doGetBaseThemes(array $themes, $theme, $used_themes = array()) {
     if (!isset($themes[$theme]->info['base theme'])) {
       return array();
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index 9a37ffc..066888a 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -2452,6 +2452,8 @@ function system_rebuild_module_data() {
  *
  * @return
  *   An associative array of themes information.
+ *
+ * @deprecated as of Drupal 8.0. Use \Drupal::service('theme_handler')->rebuildThemeData().
  */
 function _system_rebuild_theme_data() {
   return \Drupal::service('theme_handler')->rebuildThemeData();