diff --git a/core/core.services.yml b/core/core.services.yml
index 6568923..10cd1e5 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -663,7 +663,7 @@ services:
   feed.writer.wellformedwebrendererentry:
     class: Zend\Feed\Writer\Extension\WellFormedWeb\Renderer\Entry
   theme.initialisation:
-    class: Drupal\Core\Theme\Initialisation
+    class: Drupal\Core\Theme\Initialization
     arguments: ['@theme_handler', '@theme.negotiator']
     calls:
       - [setRequest, ['@?request=']]
diff --git a/core/lib/Drupal/Core/Theme/Initialization.php b/core/lib/Drupal/Core/Theme/Initialization.php
index 99da43e..22ce3f9 100644
--- a/core/lib/Drupal/Core/Theme/Initialization.php
+++ b/core/lib/Drupal/Core/Theme/Initialization.php
@@ -11,7 +11,7 @@
 use Symfony\Component\HttpFoundation\Request;
 
 /**
- * Provides the theme initialisation logic.
+ * Provides the theme initialization logic.
  */
 class Initialization {
 
@@ -59,6 +59,11 @@ public function setRequest(Request $request) {
     $this->request = $request;
   }
 
+  /**
+   * Initializes the theme system.
+   *
+   * This method uses the theme negotiator to figure out the active theme.
+   */
   public function initThemeSystem() {
     global $theme, $theme_key;
 
@@ -84,8 +89,7 @@ public function initThemeSystem() {
       $ancestor = $themes[$ancestor]->base_theme;
       $base_theme[] = $themes[$ancestor];
     }
-    $this->addAssets($themes[$theme], array_reverse($base_theme));
-    $this->loadIncludes($themes[$theme], array_reverse($base_theme));
+    $this->initThemeSystemWithLoadedInformation($themes[$theme], array_reverse($base_theme));
   }
 
   /**
@@ -93,7 +97,7 @@ public function initThemeSystem() {
    *
    * This function is useful to initialize a theme when no database is present.
    *
-   * @param $theme
+   * @param \stdClass $theme
    *   An object with the following information:
    *     filename
    *       The .info.yml file for this theme. The 'path' to
@@ -105,14 +109,30 @@ public function initThemeSystem() {
    *       The primary stylesheet for the theme. (Optional)
    *     engine
    *       The name of theme engine to use. (Optional)
-   * @param $base_theme
+   * @param \stdClass[] $base_theme
    *    An optional array of objects that represent the 'base theme' if the
    *    theme is meant to be derivative of another theme. It requires
    *    the same information as the $theme object. It should be in
    *    'oldest first' order, meaning the top level of the chain will
    *    be first.
    */
-  public function addAssets($theme, $base_theme = array()) {
+  public function initThemeSystemWithLoadedInformation($theme, $base_theme = array()) {
+    $this->addAssets($theme, $base_theme);
+    $this->loadIncludes($theme, $base_theme);
+  }
+
+  /**
+   * Adds all assets onto the response.
+   *
+   * @param \stdClass $theme
+   *   The theme object
+   *
+   * @param \stdClass[] $base_theme
+   *   A list of theme objects of all base themes.
+   *
+   * @see \Drupal\Core\Theme\Initialization::initThemeSystemWithLoadedInformation()
+   */
+  protected function addAssets($theme, $base_theme = array()) {
     global $theme_info, $base_theme_info, $theme_path;
     $theme_info = $theme;
     $base_theme_info = $base_theme;
@@ -224,12 +244,17 @@ public function addAssets($theme, $base_theme = array()) {
   }
 
   /**
+   * Load the .theme file as well as the theme engine.
+   *
    * @param \stdClass $theme
-   * @param \stdClass $base_theme
+   *   The theme object.
+   *
+   * @param \stdClass[] $base_theme
+   *   A list of theme objects of all base themes.
    *
-   * @return string
+   * @see \Drupal\Core\Theme\Initialization::initThemeSystemWithLoadedInformation()
    */
-  public function loadIncludes($theme, $base_theme) {
+  protected function loadIncludes($theme, array $base_theme = array()) {
     global $theme_engine;
 
     $theme_engine = NULL;
@@ -263,7 +288,6 @@ public function loadIncludes($theme, $base_theme) {
 
     // Always include Twig as the default theme engine.
     include_once DRUPAL_ROOT . '/core/themes/engines/twig/twig.engine';
-    return $theme_engine;
   }
 
 }