diff --git a/core/modules/system/src/Tests/Theme/EngineTwigTest.php b/core/modules/system/tests/src/Functional/Theme/EngineTwigTest.php
similarity index 97%
rename from core/modules/system/src/Tests/Theme/EngineTwigTest.php
rename to core/modules/system/tests/src/Functional/Theme/EngineTwigTest.php
index ffff75f..8c15204 100644
--- a/core/modules/system/src/Tests/Theme/EngineTwigTest.php
+++ b/core/modules/system/tests/src/Functional/Theme/EngineTwigTest.php
@@ -1,17 +1,17 @@
 <?php
 
-namespace Drupal\system\Tests\Theme;
+namespace Drupal\Tests\system\Functional\Theme;
 
 use Drupal\Core\Render\Markup;
 use Drupal\Core\Url;
-use Drupal\simpletest\WebTestBase;
+use Drupal\Tests\BrowserTestBase;
 
 /**
  * Tests Twig-specific theme functionality.
  *
  * @group Theme
  */
-class EngineTwigTest extends WebTestBase {
+class EngineTwigTest extends BrowserTestBase {
 
   /**
    * Modules to enable.
diff --git a/core/modules/system/src/Tests/Theme/EntityFilteringThemeTest.php b/core/modules/system/tests/src/Functional/Theme/EntityFilteringThemeTest.php
similarity index 96%
rename from core/modules/system/src/Tests/Theme/EntityFilteringThemeTest.php
rename to core/modules/system/tests/src/Functional/Theme/EntityFilteringThemeTest.php
index 10eda61..426e77c 100644
--- a/core/modules/system/src/Tests/Theme/EntityFilteringThemeTest.php
+++ b/core/modules/system/tests/src/Functional/Theme/EntityFilteringThemeTest.php
@@ -1,15 +1,15 @@
 <?php
 
-namespace Drupal\system\Tests\Theme;
+namespace Drupal\Tests\system\Functional\Theme;
 
 use Drupal\comment\Tests\CommentTestTrait;
 use Drupal\Core\Extension\ExtensionDiscovery;
 use Drupal\comment\CommentInterface;
 use Drupal\comment\Plugin\Field\FieldType\CommentItemInterface;
 use Drupal\node\NodeInterface;
-use Drupal\simpletest\WebTestBase;
 use Drupal\comment\Entity\Comment;
 use Drupal\taxonomy\Entity\Term;
+use Drupal\Tests\BrowserTestBase;
 
 /**
  * Tests themed output for each entity type in all available themes to ensure
@@ -17,7 +17,7 @@
  *
  * @group Theme
  */
-class EntityFilteringThemeTest extends WebTestBase {
+class EntityFilteringThemeTest extends BrowserTestBase {
 
   use CommentTestTrait;
 
diff --git a/core/modules/system/src/Tests/Theme/FunctionsTest.php b/core/modules/system/tests/src/Functional/Theme/FunctionsTest.php
similarity index 99%
rename from core/modules/system/src/Tests/Theme/FunctionsTest.php
rename to core/modules/system/tests/src/Functional/Theme/FunctionsTest.php
index 8258348..a04a0fa 100644
--- a/core/modules/system/src/Tests/Theme/FunctionsTest.php
+++ b/core/modules/system/tests/src/Functional/Theme/FunctionsTest.php
@@ -1,20 +1,20 @@
 <?php
 
-namespace Drupal\system\Tests\Theme;
+namespace Drupal\Tests\system\Functional\Theme;
 
 use Drupal\Component\Serialization\Json;
 use Drupal\Component\Utility\Html;
 use Drupal\Component\Utility\SafeMarkup;
 use Drupal\Core\Session\UserSession;
 use Drupal\Core\Url;
-use Drupal\simpletest\WebTestBase;
+use Drupal\Tests\BrowserTestBase;
 
 /**
  * Tests for common theme functions.
  *
  * @group Theme
  */
-class FunctionsTest extends WebTestBase {
+class FunctionsTest extends BrowserTestBase {
 
   /**
    * Modules to enable.
diff --git a/core/modules/system/src/Tests/Theme/HtmlAttributesTest.php b/core/modules/system/tests/src/Functional/Theme/HtmlAttributesTest.php
similarity index 86%
rename from core/modules/system/src/Tests/Theme/HtmlAttributesTest.php
rename to core/modules/system/tests/src/Functional/Theme/HtmlAttributesTest.php
index b2018c7..b79595b 100644
--- a/core/modules/system/src/Tests/Theme/HtmlAttributesTest.php
+++ b/core/modules/system/tests/src/Functional/Theme/HtmlAttributesTest.php
@@ -1,15 +1,15 @@
 <?php
 
-namespace Drupal\system\Tests\Theme;
+namespace Drupal\Tests\system\Functional\Theme;
 
-use Drupal\simpletest\WebTestBase;
+use Drupal\Tests\BrowserTestBase;
 
 /**
  * Tests attributes inserted in the 'html' and 'body' elements on the page.
  *
  * @group Theme
  */
-class HtmlAttributesTest extends WebTestBase {
+class HtmlAttributesTest extends BrowserTestBase {
 
   /**
    * Modules to enable.
diff --git a/core/modules/system/src/Tests/Theme/ThemeSuggestionsAlterTest.php b/core/modules/system/tests/src/Functional/Theme/ThemeSuggestionsAlterTest.php
similarity index 98%
rename from core/modules/system/src/Tests/Theme/ThemeSuggestionsAlterTest.php
rename to core/modules/system/tests/src/Functional/Theme/ThemeSuggestionsAlterTest.php
index c01a94f..835e24c 100644
--- a/core/modules/system/src/Tests/Theme/ThemeSuggestionsAlterTest.php
+++ b/core/modules/system/tests/src/Functional/Theme/ThemeSuggestionsAlterTest.php
@@ -1,16 +1,16 @@
 <?php
 
-namespace Drupal\system\Tests\Theme;
+namespace Drupal\Tests\system\Functional\Theme;
 
 use Drupal\Component\Utility\Xss;
-use Drupal\simpletest\WebTestBase;
+use Drupal\Tests\BrowserTestBase;
 
 /**
  * Tests theme suggestion alter hooks.
  *
  * @group Theme
  */
-class ThemeSuggestionsAlterTest extends WebTestBase {
+class ThemeSuggestionsAlterTest extends BrowserTestBase {
 
   /**
    * Modules to enable.
diff --git a/core/modules/system/src/Tests/Theme/ThemeTest.php b/core/modules/system/tests/src/Functional/Theme/ThemeTest.php
similarity index 99%
rename from core/modules/system/src/Tests/Theme/ThemeTest.php
rename to core/modules/system/tests/src/Functional/Theme/ThemeTest.php
index 64f330e..7ffe856 100644
--- a/core/modules/system/src/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/tests/src/Functional/Theme/ThemeTest.php
@@ -1,10 +1,10 @@
 <?php
 
-namespace Drupal\system\Tests\Theme;
+namespace Drupal\Tests\system\Functional\Theme;
 
 use Drupal\Component\Serialization\Json;
-use Drupal\simpletest\WebTestBase;
 use Drupal\test_theme\ThemeClass;
+use Drupal\Tests\BrowserTestBase;
 use Symfony\Cmf\Component\Routing\RouteObjectInterface;
 use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\Routing\Route;
@@ -15,7 +15,7 @@
  *
  * @group Theme
  */
-class ThemeTest extends WebTestBase {
+class ThemeTest extends BrowserTestBase {
 
   /**
    * Modules to enable.
diff --git a/core/modules/system/src/Tests/Theme/TwigDebugMarkupTest.php b/core/modules/system/tests/src/Functional/Theme/TwigDebugMarkupTest.php
similarity index 97%
rename from core/modules/system/src/Tests/Theme/TwigDebugMarkupTest.php
rename to core/modules/system/tests/src/Functional/Theme/TwigDebugMarkupTest.php
index 457fbad..171044e 100644
--- a/core/modules/system/src/Tests/Theme/TwigDebugMarkupTest.php
+++ b/core/modules/system/tests/src/Functional/Theme/TwigDebugMarkupTest.php
@@ -1,15 +1,15 @@
 <?php
 
-namespace Drupal\system\Tests\Theme;
+namespace Drupal\Tests\system\Functional\Theme;
 
-use Drupal\simpletest\WebTestBase;
+use Drupal\Tests\BrowserTestBase;
 
 /**
  * Tests for Twig debug markup.
  *
  * @group Theme
  */
-class TwigDebugMarkupTest extends WebTestBase {
+class TwigDebugMarkupTest extends BrowserTestBase {
 
   /**
    * Modules to enable.
diff --git a/core/modules/system/src/Tests/Theme/TwigFilterTest.php b/core/modules/system/tests/src/Functional/Theme/TwigFilterTest.php
similarity index 97%
rename from core/modules/system/src/Tests/Theme/TwigFilterTest.php
rename to core/modules/system/tests/src/Functional/Theme/TwigFilterTest.php
index 653e4fa..8fe8a8e 100644
--- a/core/modules/system/src/Tests/Theme/TwigFilterTest.php
+++ b/core/modules/system/tests/src/Functional/Theme/TwigFilterTest.php
@@ -1,15 +1,15 @@
 <?php
 
-namespace Drupal\system\Tests\Theme;
+namespace Drupal\Tests\system\Functional\Theme;
 
-use Drupal\simpletest\WebTestBase;
+use Drupal\Tests\BrowserTestBase;
 
 /**
  * Tests Drupal's Twig filters.
  *
  * @group Theme
  */
-class TwigFilterTest extends WebTestBase {
+class TwigFilterTest extends BrowserTestBase {
 
   /**
    * Modules to enable.
diff --git a/core/modules/system/src/Tests/Theme/TwigNamespaceTest.php b/core/modules/system/tests/src/Functional/Theme/TwigNamespaceTest.php
similarity index 92%
rename from core/modules/system/src/Tests/Theme/TwigNamespaceTest.php
rename to core/modules/system/tests/src/Functional/Theme/TwigNamespaceTest.php
index ab62889..3d3294d 100644
--- a/core/modules/system/src/Tests/Theme/TwigNamespaceTest.php
+++ b/core/modules/system/tests/src/Functional/Theme/TwigNamespaceTest.php
@@ -1,15 +1,15 @@
 <?php
 
-namespace Drupal\system\Tests\Theme;
+namespace Drupal\Tests\system\Functional\Theme;
 
-use Drupal\simpletest\WebTestBase;
+use Drupal\Tests\BrowserTestBase;
 
 /**
  * Tests Twig namespaces.
  *
  * @group Theme
  */
-class TwigNamespaceTest extends WebTestBase {
+class TwigNamespaceTest extends BrowserTestBase {
 
   /**
    * Modules to enable.
diff --git a/core/modules/system/src/Tests/Theme/TwigRawTest.php b/core/modules/system/tests/src/Functional/Theme/TwigRawTest.php
similarity index 90%
rename from core/modules/system/src/Tests/Theme/TwigRawTest.php
rename to core/modules/system/tests/src/Functional/Theme/TwigRawTest.php
index 8be63db..17b7194 100644
--- a/core/modules/system/src/Tests/Theme/TwigRawTest.php
+++ b/core/modules/system/tests/src/Functional/Theme/TwigRawTest.php
@@ -1,15 +1,15 @@
 <?php
 
-namespace Drupal\system\Tests\Theme;
+namespace Drupal\Tests\system\Functional\Theme;
 
-use Drupal\simpletest\WebTestBase;
+use Drupal\Tests\BrowserTestBase;
 
 /**
  * Tests Twig 'raw' filter.
  *
  * @group Theme
  */
-class TwigRawTest extends WebTestBase {
+class TwigRawTest extends BrowserTestBase {
 
   /**
    * Modules to enable.
diff --git a/core/modules/system/src/Tests/Theme/TwigTransTest.php b/core/modules/system/tests/src/Functional/Theme/TwigTransTest.php
similarity index 98%
rename from core/modules/system/src/Tests/Theme/TwigTransTest.php
rename to core/modules/system/tests/src/Functional/Theme/TwigTransTest.php
index b73cdaf..4c7bca2 100644
--- a/core/modules/system/src/Tests/Theme/TwigTransTest.php
+++ b/core/modules/system/tests/src/Functional/Theme/TwigTransTest.php
@@ -1,16 +1,16 @@
 <?php
 
-namespace Drupal\system\Tests\Theme;
+namespace Drupal\Tests\system\Functional\Theme;
 
 use Drupal\Core\Language\LanguageInterface;
-use Drupal\simpletest\WebTestBase;
+use Drupal\Tests\BrowserTestBase;
 
 /**
  * Tests Twig "trans" tags.
  *
  * @group Theme
  */
-class TwigTransTest extends WebTestBase {
+class TwigTransTest extends BrowserTestBase {
 
   /**
    * Modules to enable.