diff --git a/core/config/install/core.extension.yml b/core/config/install/core.extension.yml
index eae39ef..1514a9e 100644
--- a/core/config/install/core.extension.yml
+++ b/core/config/install/core.extension.yml
@@ -1,5 +1,4 @@
 module: {}
-theme:
-  stark: 0
+theme: {}
 disabled:
   theme: {}
diff --git a/core/modules/simpletest/src/KernelTestBase.php b/core/modules/simpletest/src/KernelTestBase.php
index afea590..616d064 100644
--- a/core/modules/simpletest/src/KernelTestBase.php
+++ b/core/modules/simpletest/src/KernelTestBase.php
@@ -176,7 +176,7 @@ protected function setUp() {
     // \Drupal\Core\Config\ConfigInstaller::installDefaultConfig() to work.
     // Write directly to active storage to avoid early instantiation of
     // the event dispatcher which can prevent modules from registering events.
-    \Drupal::service('config.storage')->write('core.extension', array('module' => array(), 'theme' => array('seven' => 1, 'stark' => 1)));
+    \Drupal::service('config.storage')->write('core.extension', array('module' => array(), 'theme' => array()));
 
     // Collect and set a fixed module list.
     $class = get_class($this);