diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index ae6d1be..e84c095 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -716,15 +716,12 @@ function list_themes($refresh = FALSE) {
  *   An array of available themes.
  * @param $key
  *   The name of the theme whose base we are looking for.
- * @param $used_keys
- *   (optional) A recursion parameter preventing endless loops. Defaults to
- *   NULL.
  *
  * @return
  *   Returns an array of all of the theme's ancestors; the first element's value
  *   will be NULL if an error occurred.
  */
-function drupal_find_base_themes($themes, $key, $used_keys = array()) {
+function drupal_find_base_themes($themes, $key) {
   return \Drupal::service('theme_handler')->getBaseThemes($themes, $key);
 }
 
diff --git a/core/lib/Drupal/Core/Theme/ThemeHandler.php b/core/lib/Drupal/Core/Theme/ThemeHandler.php
index e8f6249..8adea15 100644
--- a/core/lib/Drupal/Core/Theme/ThemeHandler.php
+++ b/core/lib/Drupal/Core/Theme/ThemeHandler.php
@@ -18,10 +18,15 @@
 use Drupal\Core\Extension\InfoParserInterface;
 
 /**
- * Manages the list of available themes as well as enable/disable them.
+ * Default theme handler using the config system for enabled/disabled themes.
  */
 class ThemeHandler implements ThemeHandlerInterface {
 
+  /**
+   * Contains the features enabled for themes by default.
+   *
+   * @var array
+   */
   protected $defaultFeatures = array(
     'logo',
     'favicon',
@@ -90,7 +95,9 @@ class ThemeHandler implements ThemeHandlerInterface {
    * @param \Drupal\Core\Extension\InfoParserInterface $info_parser
    *   The info parser to parse the theme.info.yml files.
    * @param \Drupal\Core\Routing\RouteBuilder $route_builder
-   *   The route builder to rebuild the routes if a theme is enabled.
+   *   (optional) The route builder to rebuild the routes if a theme is enabled.
+   * @param \Drupal\Core\SystemListingInfo $system_list_info
+   *   (optional) The system listing info.
    */
   public function __construct(ConfigFactory $config_factory, ModuleHandlerInterface $module_handler, CacheBackendInterface $cache_backend, InfoParserInterface $info_parser, RouteBuilder $route_builder = NULL, SystemListingInfo $system_list_info = NULL) {
     $this->configFactory = $config_factory;
@@ -350,18 +357,18 @@ public function rebuildThemeData() {
    * e.g., to prefix all values of the 'stylesheets' or 'scripts' properties with
    * the file path to the defining module/theme.
    *
-   * @param $info
+   * @param array $info
    *   A nested array of data of an .info.yml file to be processed.
-   * @param $path
+   * @param string $path
    *   A file path to prepend to each value in $info.
    *
-   * @return
+   * @return array
    *   The $info array with prefixed values.
    *
    * @see _system_rebuild_module_data()
-   * @see _system_rebuild_theme_data()
+   * @see self::rebuildThemeData()
    */
-  protected function themeInfoPrefixPath($info, $path) {
+  protected function themeInfoPrefixPath(array $info, $path) {
     foreach ($info as $key => $value) {
       // Recurse into nested values until we reach the deepest level.
       if (is_array($value)) {
@@ -430,6 +437,9 @@ protected function configInstallDefaultConfig($theme) {
     config_install_default_config('theme', $theme);
   }
 
+  /**
+   * Resets some other systems like rebuilding the route information or caches.
+   */
   protected function resetSystem() {
     if ($this->routeBuilder) {
       $this->routeBuilder->rebuild();
@@ -442,18 +452,32 @@ protected function resetSystem() {
     $this->themeRegistryRebuild();
   }
 
+  /**
+   * Wraps system_list_reset().
+   */
   protected function systemListReset() {
     system_list_reset();
   }
 
+  /**
+   * Wraps drupal_clear_css_cache().
+   */
   protected function clearCssCache() {
     drupal_clear_css_cache();
   }
 
+  /**
+   * Wraps drupal_theme_rebuild().
+   */
   protected function themeRegistryRebuild() {
     drupal_theme_rebuild();
   }
 
+  /**
+   * Wraps system_list().
+   *
+   * @return array
+   */
   protected function systemThemeList() {
     return system_list('theme');
   }
diff --git a/core/lib/Drupal/Core/Theme/ThemeHandlerInterface.php b/core/lib/Drupal/Core/Theme/ThemeHandlerInterface.php
index 5314f96..fb7c850 100644
--- a/core/lib/Drupal/Core/Theme/ThemeHandlerInterface.php
+++ b/core/lib/Drupal/Core/Theme/ThemeHandlerInterface.php
@@ -18,7 +18,8 @@
    * @param array $theme_list
    *   An array of theme names.
    *
-   * @throws ExtensionNameLengthException
+   * @throws \Drupal\Core\Extension\ExtensionNameLengthException
+   *   Thrown when the theme name is to long
    */
   public function enable(array $theme_list);
 
@@ -33,8 +34,8 @@ public function disable(array $theme_list);
   /**
    * Returns a list of all currently available themes.
    *
-   * Retrieved from the database, if available and the site is not in maintenance
-   * mode; otherwise compiled freshly from the filesystem.
+   * Retrieved from the database, if available and the site is not in
+   * maintenance mode; otherwise compiled freshly from the filesystem.
    *
    * @return array
    *   An associative array of the currently available themes. The keys are the
@@ -50,8 +51,8 @@ public function disable(array $theme_list);
    *     themes/bartik/style.css). Not set if no stylesheets are defined in the
    *     .info.yml file.
    *   - scripts: An associative array of JavaScripts, using the filename as key
-   *     and the complete filepath as value. Not set if no scripts are defined in
-   *     the .info.yml file.
+   *     and the complete filepath as value. Not set if no scripts are defined
+   *     in the .info.yml file.
    *   - prefix: The base theme engine prefix.
    *   - engine: The machine name of the theme engine.
    *   - base_theme: If this is a sub-theme, the machine name of the base theme