diff --git a/core/modules/block/src/Tests/BlockTest.php b/core/modules/block/src/Tests/BlockTest.php
index c98b042..57c1633 100644
--- a/core/modules/block/src/Tests/BlockTest.php
+++ b/core/modules/block/src/Tests/BlockTest.php
@@ -199,7 +199,7 @@ public function testBlockThemeSelector() {
     // Install all themes.
     \Drupal::service('theme_handler')->install(array('bartik', 'seven'));
     $theme_settings = $this->config('system.theme');
-    foreach (array('bartik', 'classy', 'seven') as $theme) {
+    foreach (array('bartik', 'test_classy', 'seven') as $theme) {
       $this->drupalGet('admin/structure/block/list/' . $theme);
       $this->assertTitle(t('Block layout') . ' | Drupal');
       // Select the 'Powered by Drupal' block to be placed.
@@ -442,7 +442,7 @@ public function testUninstallTheme() {
     $this->drupalGet('<front>');
     $this->assertText('Powered by Drupal');
 
-    $theme_handler->setDefault('classy');
+    $theme_handler->setDefault('test_classy');
     $theme_handler->uninstall(['seven']);
 
     // Ensure that the block configuration does not exist anymore.
diff --git a/core/modules/block/src/Tests/Views/DisplayBlockTest.php b/core/modules/block/src/Tests/Views/DisplayBlockTest.php
index f7b3ffe..542567f 100644
--- a/core/modules/block/src/Tests/Views/DisplayBlockTest.php
+++ b/core/modules/block/src/Tests/Views/DisplayBlockTest.php
@@ -70,7 +70,7 @@ public function testBlockCategory() {
     $arguments = array(
       ':href' => \Drupal::Url('block.admin_add', array(
         'plugin_id' => 'views_block:' . $edit['id'] . '-block_1',
-        'theme' => 'classy',
+        'theme' => 'test_classy',
       )),
       ':category' => t('Lists (Views)'),
     );
@@ -107,7 +107,7 @@ public function testBlockCategory() {
     $arguments = array(
       ':href' => \Drupal::Url('block.admin_add', array(
         'plugin_id' => 'views_block:' . $edit['id'] . '-block_2',
-        'theme' => 'classy',
+        'theme' => 'test_classy',
       )),
       ':category' => t('Lists (Views)'),
     );
@@ -117,7 +117,7 @@ public function testBlockCategory() {
     $arguments = array(
       ':href' => \Drupal::Url('block.admin_add', array(
         'plugin_id' => 'views_block:' . $edit['id'] . '-block_3',
-        'theme' => 'classy',
+        'theme' => 'test_classy',
       )),
       ':category' => $category,
     );
diff --git a/core/modules/config/tests/config_override_integration_test/config/install/block.block.config_override_test.yml b/core/modules/config/tests/config_override_integration_test/config/install/block.block.config_override_test.yml
index be0616f..0025b84 100644
--- a/core/modules/config/tests/config_override_integration_test/config/install/block.block.config_override_test.yml
+++ b/core/modules/config/tests/config_override_integration_test/config/install/block.block.config_override_test.yml
@@ -1,5 +1,5 @@
 id: config_override_test
-theme: classy
+theme: test_classy
 weight: 0
 status: true
 langcode: en
@@ -16,7 +16,7 @@ dependencies:
   module:
     - block_test
   theme:
-    - classy
+    - test_classy
 visibility:
   request_path:
     id: request_path
diff --git a/core/modules/field/src/Tests/EntityReference/EntityReferenceFormatterTest.php b/core/modules/field/src/Tests/EntityReference/EntityReferenceFormatterTest.php
index cc41f7d..1cc3b38 100644
--- a/core/modules/field/src/Tests/EntityReference/EntityReferenceFormatterTest.php
+++ b/core/modules/field/src/Tests/EntityReference/EntityReferenceFormatterTest.php
@@ -65,8 +65,8 @@ protected function setUp() {
     parent::setUp();
 
     // Use Classy theme for testing markup output.
-    \Drupal::service('theme_handler')->install(['classy']);
-    $this->config('system.theme')->set('default', 'classy')->save();
+    \Drupal::service('theme_handler')->install(['test_classy']);
+    $this->config('system.theme')->set('default', 'test_classy')->save();
 
     // Grant the 'view test entity' permission.
     $this->installConfig(array('user'));
diff --git a/core/modules/rdf/src/Tests/Field/NumberFieldRdfaTest.php b/core/modules/rdf/src/Tests/Field/NumberFieldRdfaTest.php
index 120d6f0..a450030 100644
--- a/core/modules/rdf/src/Tests/Field/NumberFieldRdfaTest.php
+++ b/core/modules/rdf/src/Tests/Field/NumberFieldRdfaTest.php
@@ -32,8 +32,8 @@ public function testIntegerFormatter() {
    * Tests the integer formatter with settings.
    */
   public function testIntegerFormatterWithSettings() {
-    \Drupal::service('theme_handler')->install(['classy']);
-    $this->config('system.theme')->set('default', 'classy')->save();
+    \Drupal::service('theme_handler')->install(['test_classy']);
+    $this->config('system.theme')->set('default', 'test_classy')->save();
     $this->fieldType = 'integer';
     $formatter = array(
       'type' => 'number_integer',
@@ -75,8 +75,8 @@ public function testFloatFormatter() {
    * Tests the float formatter with settings.
    */
   public function testFloatFormatterWithSettings() {
-    \Drupal::service('theme_handler')->install(['classy']);
-    $this->config('system.theme')->set('default', 'classy')->save();
+    \Drupal::service('theme_handler')->install(['test_classy']);
+    $this->config('system.theme')->set('default', 'test_classy')->save();
     $this->fieldType = 'float';
     $formatter = array(
       'type' => 'number_decimal',
@@ -125,8 +125,8 @@ public function testFloatFormatterWithScale() {
    * Tests the float formatter with a scale. Scale is exercised.
    */
   public function testFloatFormatterWithScaleExercised() {
-    \Drupal::service('theme_handler')->install(['classy']);
-    $this->config('system.theme')->set('default', 'classy')->save();
+    \Drupal::service('theme_handler')->install(['test_classy']);
+    $this->config('system.theme')->set('default', 'test_classy')->save();
     $this->fieldType = 'float';
     $formatter = array(
       'type' => 'number_decimal',
@@ -163,8 +163,8 @@ public function testDecimalFormatter() {
    * Tests the decimal formatter with settings.
    */
   public function testDecimalFormatterWithSettings() {
-    \Drupal::service('theme_handler')->install(['classy']);
-    $this->config('system.theme')->set('default', 'classy')->save();
+    \Drupal::service('theme_handler')->install(['test_classy']);
+    $this->config('system.theme')->set('default', 'test_classy')->save();
     $this->fieldType = 'decimal';
     $formatter = array(
       'type' => 'number_decimal',
diff --git a/core/modules/search/src/Tests/SearchBlockTest.php b/core/modules/search/src/Tests/SearchBlockTest.php
index 8351878..2d3d62f 100644
--- a/core/modules/search/src/Tests/SearchBlockTest.php
+++ b/core/modules/search/src/Tests/SearchBlockTest.php
@@ -37,7 +37,7 @@ public function testSearchFormBlock() {
     // Test availability of the search block in the admin "Place blocks" list.
     $this->drupalGet('admin/structure/block');
     $this->clickLinkPartialName('Place block');
-    $this->assertLinkByHref('/admin/structure/block/add/search_form_block/classy', 0,
+    $this->assertLinkByHref('/admin/structure/block/add/search_form_block/test_classy', 0,
       'Did not find the search block in block candidate list.');
 
     $block = $this->drupalPlaceBlock('search_form_block');
diff --git a/core/modules/system/src/Tests/Render/Element/TableTest.php b/core/modules/system/src/Tests/Render/Element/TableTest.php
index 725f339..665ac2e 100644
--- a/core/modules/system/src/Tests/Render/Element/TableTest.php
+++ b/core/modules/system/src/Tests/Render/Element/TableTest.php
@@ -97,8 +97,8 @@ function testThemeTableWithEmptyMessage() {
     );
 
     // Enable the Classy theme.
-    \Drupal::service('theme_handler')->install(['classy']);
-    $this->config('system.theme')->set('default', 'classy')->save();
+    \Drupal::service('theme_handler')->install(['test_classy']);
+    $this->config('system.theme')->set('default', 'test_classy')->save();
 
     $this->render($table);
     $this->removeWhiteSpace();
diff --git a/core/modules/system/src/Tests/Theme/MessageTest.php b/core/modules/system/src/Tests/Theme/MessageTest.php
index db27d2a..6c577b7 100644
--- a/core/modules/system/src/Tests/Theme/MessageTest.php
+++ b/core/modules/system/src/Tests/Theme/MessageTest.php
@@ -26,8 +26,8 @@ class MessageTest extends KernelTestBase {
    */
   function testMessages() {
     // Enable the Classy theme.
-    \Drupal::service('theme_handler')->install(['classy']);
-    $this->config('system.theme')->set('default', 'classy')->save();
+    \Drupal::service('theme_handler')->install(['test_classy']);
+    $this->config('system.theme')->set('default', 'test_classy')->save();
 
     drupal_set_message('An error occurred', 'error');
     drupal_set_message('But then something nice happened');
diff --git a/core/modules/system/tests/modules/theme_test/src/Theme/HighPriorityThemeNegotiator.php b/core/modules/system/tests/modules/theme_test/src/Theme/HighPriorityThemeNegotiator.php
index 7934ae3..70fcc24 100644
--- a/core/modules/system/tests/modules/theme_test/src/Theme/HighPriorityThemeNegotiator.php
+++ b/core/modules/system/tests/modules/theme_test/src/Theme/HighPriorityThemeNegotiator.php
@@ -26,7 +26,7 @@ public function applies(RouteMatchInterface $route_match) {
    * {@inheritdoc}
    */
   public function determineActiveTheme(RouteMatchInterface $route_match) {
-    return 'classy';
+    return 'test_classy';
   }
 
 }