diff --git a/core/lib/Drupal/Core/DrupalKernel.php b/core/lib/Drupal/Core/DrupalKernel.php
index 37ed0e97a6..829af6cdb3 100644
--- a/core/lib/Drupal/Core/DrupalKernel.php
+++ b/core/lib/Drupal/Core/DrupalKernel.php
@@ -585,6 +585,9 @@ public function discoverServiceProviders() {
     if (!isset($this->moduleList)) {
       $extensions = $this->getConfigStorage()->read('core.extension');
       $this->moduleList = isset($extensions['module']) ? $extensions['module'] : [];
+      if (isset($extensions['theme'])) {
+        $this->moduleList = array_merge($this->moduleList, $extensions['theme']);
+      }
     }
     $module_filenames = $this->getModuleFileNames();
     $this->classLoaderAddMultiplePsr4($this->getModuleNamespacesPsr4($module_filenames));
@@ -754,6 +757,7 @@ protected function moduleData($module) {
 
       // Now find modules.
       $this->moduleData = $profiles + $listing->scan('module');
+      $this->moduleData = array_merge($this->moduleData, $listing->scan('theme'));
     }
     return isset($this->moduleData[$module]) ? $this->moduleData[$module] : FALSE;
   }