diff --git a/core/modules/system/src/Tests/Theme/ThemeInfoTest.php b/core/modules/system/src/Tests/Theme/ThemeInfoTest.php
index 043986a..0763917 100644
--- a/core/modules/system/src/Tests/Theme/ThemeInfoTest.php
+++ b/core/modules/system/src/Tests/Theme/ThemeInfoTest.php
@@ -84,7 +84,7 @@ function testStylesheets() {
    */
   public function testChanges() {
     $this->themeHandler->install(array('test_theme'));
-    $this->themeHandler->setDefault('test_theme');
+    $this->config('system.theme')->set('default', 'test_theme')->save();
     $this->themeManager->resetActiveTheme();
 
     $active_theme = $this->themeManager->getActiveTheme();
diff --git a/core/tests/Drupal/KernelTests/Core/Theme/StableThemeTest.php b/core/tests/Drupal/KernelTests/Core/Theme/StableThemeTest.php
index da585d0..8a79a9e 100644
--- a/core/tests/Drupal/KernelTests/Core/Theme/StableThemeTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Theme/StableThemeTest.php
@@ -47,7 +47,7 @@ protected function setUp() {
    */
   public function testStableIsDefault() {
     $this->themeHandler->install(['test_stable']);
-    $this->themeHandler->setDefault('test_stable');
+    $this->config('system.theme')->set('default', 'test_stable')->save();
     $theme = $this->themeManager->getActiveTheme();
     /** @var \Drupal\Core\Theme\ActiveTheme $base_theme */
     $base_themes = $theme->getBaseThemes();
@@ -60,7 +60,7 @@ public function testStableIsDefault() {
    */
   public function testWildWest() {
     $this->themeHandler->install(['test_wild_west']);
-    $this->themeHandler->setDefault('test_wild_west');
+    $this->config('system.theme')->set('default', 'test_wild_west')->save();
     $theme = $this->themeManager->getActiveTheme();
     /** @var \Drupal\Core\Theme\ActiveTheme $base_theme */
     $base_themes = $theme->getBaseThemes();
diff --git a/core/tests/Drupal/KernelTests/Core/Theme/ThemeInstallerTest.php b/core/tests/Drupal/KernelTests/Core/Theme/ThemeInstallerTest.php
index b82978c..f3e2ade 100644
--- a/core/tests/Drupal/KernelTests/Core/Theme/ThemeInstallerTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Theme/ThemeInstallerTest.php
@@ -145,7 +145,7 @@ function testUninstallDefault() {
     $name = 'stark';
     $other_name = 'bartik';
     $this->themeInstaller()->install(array($name, $other_name));
-    $this->themeHandler()->setDefault($name);
+    $this->config('system.theme')->set('default', $name)->save();
 
     $themes = $this->themeHandler()->listInfo();
     $this->assertTrue(isset($themes[$name]));