diff -u b/core/lib/Drupal/Core/Theme/ThemeInitialization.php b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
--- b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
+++ b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
@@ -54,12 +54,13 @@
    *   The theme handler.
    * @param \Drupal\Core\State\StateInterface $state
    *   The state.
+   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
+   *   The module handler to use to load modules.
    */
   public function __construct($root, ThemeHandlerInterface $theme_handler, StateInterface $state, ModuleHandlerInterface $module_handler) {
     $this->root = $root;
     $this->themeHandler = $theme_handler;
     $this->state = $state;
-
     $this->moduleHandler = $module_handler;
   }
 
@@ -111,7 +112,6 @@
       $base_themes[] = $themes[$ancestor];
     }
 
-    $this->prepareExtensions();
     $active_theme = $this->getActiveTheme($themes[$theme_name], $base_themes);
 
     $this->state->set('theme.active_theme.' . $theme_name, $active_theme);
@@ -223,7 +223,7 @@
    *
    * @return array
    */
-  protected function prepareExtensions() {
+  protected function getExtensions() {
     if (!isset($this->extensions)) {
       $this->extensions = array_merge($this->moduleHandler->getModuleList(),  $this->themeHandler->listInfo());
     }
@@ -231,12 +231,13 @@
   }
 
   /**
-   * Resolves tokens from the stylesheets remove.
+   * Gets CSS file where tokens have been resolved.
    *
-   * @param array $list
+   * @param string $css_file
+   *   CSS file which may contain tokens.
    *
-   * @return array
-   *   List of stylesheets where placeholders are replaced.
+   * @return string
+   *   CSS file where placeholders are replaced.
    */
   protected function resolveStyleSheetPlaceholders($css_file) {
     $token_candidate = explode('/', $css_file)[0];
@@ -247,7 +248,7 @@
     $token = substr($token_candidate, 1);
 
     if (isset($this->extensions[$token])) {
-      return str_replace($token_candidate, $this->extensions[$token]->getPath(), $css_file);
+      return str_replace($token_candidate, $this->getExtensions()[$token]->getPath(), $css_file);
     }
   }
 }