diff --git a/core/modules/block/src/Tests/BlockTest.php b/core/modules/block/src/Tests/BlockTest.php
index 41d376a..032b87a 100644
--- a/core/modules/block/src/Tests/BlockTest.php
+++ b/core/modules/block/src/Tests/BlockTest.php
@@ -481,12 +481,12 @@ public function testUninstallTheme() {
     $theme_handler = \Drupal::service('theme_handler');
 
     $theme_handler->install(['seven']);
-    $theme_handler->setDefault('seven');
+    $this->config('system.theme')->set('default', 'seven')->save();
     $block = $this->drupalPlaceBlock('system_powered_by_block', ['theme' => 'seven', 'region' => 'help']);
     $this->drupalGet('<front>');
     $this->assertText('Powered by Drupal');
 
-    $theme_handler->setDefault('classy');
+    $this->config('system.theme')->set('default', 'classy')->save();
     $theme_handler->uninstall(['seven']);
 
     // Ensure that the block configuration does not exist anymore.
diff --git a/core/modules/ckeditor/src/Tests/CKEditorLoadingTest.php b/core/modules/ckeditor/src/Tests/CKEditorLoadingTest.php
index e14b6e3..6efe647 100644
--- a/core/modules/ckeditor/src/Tests/CKEditorLoadingTest.php
+++ b/core/modules/ckeditor/src/Tests/CKEditorLoadingTest.php
@@ -201,7 +201,7 @@ function testExternalStylesheets() {
     $theme_handler = \Drupal::service('theme_handler');
     // Case 1: Install theme which has an absolute external CSS URL.
     $theme_handler->install(['test_ckeditor_stylesheets_external']);
-    $theme_handler->setDefault('test_ckeditor_stylesheets_external');
+    $this->config('system.theme')->set('default', 'test_ckeditor_stylesheets_external')->save();
     $expected = [
       'https://fonts.googleapis.com/css?family=Open+Sans',
     ];
@@ -209,7 +209,7 @@ function testExternalStylesheets() {
 
     // Case 2: Install theme which has an external protocol-relative CSS URL.
     $theme_handler->install(['test_ckeditor_stylesheets_protocol_relative']);
-    $theme_handler->setDefault('test_ckeditor_stylesheets_protocol_relative');
+    $this->config('system.theme')->set('default', 'test_ckeditor_stylesheets_protocol_relative')->save();
     $expected = [
       '//fonts.googleapis.com/css?family=Open+Sans',
     ];
@@ -217,7 +217,7 @@ function testExternalStylesheets() {
 
     // Case 3: Install theme which has a relative CSS URL.
     $theme_handler->install(['test_ckeditor_stylesheets_relative']);
-    $theme_handler->setDefault('test_ckeditor_stylesheets_relative');
+    $this->config('system.theme')->set('default', 'test_ckeditor_stylesheets_relative')->save();
     $expected = [
       'core/modules/system/tests/themes/test_ckeditor_stylesheets_relative/css/yokotsoko.css',
     ];
diff --git a/core/modules/system/src/Tests/System/ThemeTest.php b/core/modules/system/src/Tests/System/ThemeTest.php
index e01f28a..667c97a 100644
--- a/core/modules/system/src/Tests/System/ThemeTest.php
+++ b/core/modules/system/src/Tests/System/ThemeTest.php
@@ -305,7 +305,7 @@ function testSwitchDefaultTheme() {
     $theme_handler = \Drupal::service('theme_handler');
     // First, install Stark and set it as the default theme programmatically.
     $theme_handler->install(array('stark'));
-    $theme_handler->setDefault('stark');
+    $this->config('system.theme')->set('default', 'stark')->save();
 
     // Install Bartik and set it as the default theme.
     $theme_handler->install(array('bartik'));
diff --git a/core/modules/system/src/Tests/Theme/TwigSettingsTest.php b/core/modules/system/src/Tests/Theme/TwigSettingsTest.php
index b776639..3722431 100644
--- a/core/modules/system/src/Tests/Theme/TwigSettingsTest.php
+++ b/core/modules/system/src/Tests/Theme/TwigSettingsTest.php
@@ -78,7 +78,7 @@ function testTwigCacheOverride() {
     $extension = twig_extension();
     $theme_handler = $this->container->get('theme_handler');
     $theme_handler->install(array('test_theme'));
-    $theme_handler->setDefault('test_theme');
+    $this->config('system.theme')->set('default', 'test_theme')->save();
 
     // The registry still works on theme globals, so set them here.
     \Drupal::theme()->setActiveTheme(\Drupal::service('theme.initialization')->getActiveThemeByName('test_theme'));
diff --git a/core/tests/Drupal/KernelTests/Core/Theme/RegistryTest.php b/core/tests/Drupal/KernelTests/Core/Theme/RegistryTest.php
index 93e931b..257c61e 100644
--- a/core/tests/Drupal/KernelTests/Core/Theme/RegistryTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Theme/RegistryTest.php
@@ -150,7 +150,7 @@ public function testThemeRegistryAlterByTheme() {
     /** @var \Drupal\Core\Extension\ThemeHandlerInterface $theme_handler */
     $theme_handler = \Drupal::service('theme_handler');
     $theme_handler->install(['test_theme']);
-    $theme_handler->setDefault('test_theme');
+    $this->config('system.theme')->set('default', 'test_theme')->save();
 
     $registry = new Registry(\Drupal::root(), \Drupal::cache(), \Drupal::lock(), \Drupal::moduleHandler(), $theme_handler, \Drupal::service('theme.initialization'), 'test_theme');
     $registry->setThemeManager(\Drupal::theme());