.../Drupal/Core/Test/FunctionalTestSetupTrait.php  | 42 ++++++++++++++++++++++
 .../testing/config/install/system.theme.yml        |  2 --
 core/profiles/testing/testing.info.yml             |  3 --
 core/tests/Drupal/Tests/BrowserTestBase.php        |  8 +++++
 .../Tests/Listeners/DeprecationListenerTrait.php   |  2 ++
 5 files changed, 52 insertions(+), 5 deletions(-)

diff --git a/core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php b/core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php
index 3f8a74326a..b05c05d39c 100644
--- a/core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php
+++ b/core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php
@@ -8,6 +8,8 @@
 use Drupal\Core\Config\Development\ConfigSchemaChecker;
 use Drupal\Core\Database\Database;
 use Drupal\Core\DrupalKernel;
+use Drupal\Core\Extension\Exception\UnknownExtensionException;
+use Drupal\Core\Extension\ExtensionNameLengthException;
 use Drupal\Core\Extension\MissingDependencyException;
 use Drupal\Core\File\FileSystemInterface;
 use Drupal\Core\Serialization\Yaml;
@@ -359,6 +361,20 @@ protected function initConfig(ContainerInterface $container) {
     $config->getEditable('system.date')
       ->set('timezone.default', 'Australia/Sydney')
       ->save();
+
+    if (empty($this->theme)) {
+      @trigger_error('Drupal\Tests\BrowserTestBase::$theme is required in drupal:9.0.0. See https://www.drupal.org/node/2352949', E_USER_DEPRECATED);
+      $this->theme = 'classy';
+      if ($this->profile === 'demo_umami') {
+        $this->theme = 'umami';
+      }
+      elseif ($this->profile === 'standard') {
+        $this->theme = 'bartik';
+      }
+    }
+    $config->getEditable('system.theme')
+      ->set('default', $this->theme)
+      ->save();
   }
 
   /**
@@ -403,6 +419,32 @@ protected function initKernel(Request $request) {
     return $this->kernel->getContainer();
   }
 
+  /**
+   * Install the theme defined by `static::$theme`.
+   *
+   * To install a test theme outside of the testing environment, add
+   * @code
+   * $settings['extension_discovery_scan_tests'] = TRUE;
+   * @endcode
+   * to your settings.php.
+   *
+   * @param \Symfony\Component\DependencyInjection\ContainerInterface $container
+   *   The container.
+   */
+  protected function installThemeFromClassProperty(ContainerInterface $container) {
+    try {
+      $container->get('theme_installer')->install([$this->theme], TRUE);
+    }
+    catch (UnknownExtensionException $e) {
+      // The exception message has all the details.
+      $this->fail($e->getMessage());
+    }
+    catch (ExtensionNameLengthException $e) {
+      // The exception message has all the details.
+      $this->fail($e->getMessage());
+    }
+  }
+
   /**
    * Install modules defined by `static::$modules`.
    *
diff --git a/core/profiles/testing/config/install/system.theme.yml b/core/profiles/testing/config/install/system.theme.yml
deleted file mode 100644
index 0defc7eaec..0000000000
--- a/core/profiles/testing/config/install/system.theme.yml
+++ /dev/null
@@ -1,2 +0,0 @@
-# @todo: Remove this file in https://www.drupal.org/node/2352949
-default: classy
diff --git a/core/profiles/testing/testing.info.yml b/core/profiles/testing/testing.info.yml
index 9924817a3d..834198a743 100644
--- a/core/profiles/testing/testing.info.yml
+++ b/core/profiles/testing/testing.info.yml
@@ -9,6 +9,3 @@ install:
   # tests as possible run with them enabled.
   - drupal:page_cache
   - dynamic_page_cache
-# @todo: Remove this in https://www.drupal.org/node/2352949
-themes:
-  - classy
diff --git a/core/tests/Drupal/Tests/BrowserTestBase.php b/core/tests/Drupal/Tests/BrowserTestBase.php
index 2b1ee867b1..617e326fbf 100644
--- a/core/tests/Drupal/Tests/BrowserTestBase.php
+++ b/core/tests/Drupal/Tests/BrowserTestBase.php
@@ -114,6 +114,13 @@ abstract class BrowserTestBase extends TestCase {
    */
   protected $profile = 'testing';
 
+  /**
+   * The theme to install as the default for testing.
+   *
+   * @var string
+   */
+  protected $theme;
+
   /**
    * An array of custom translations suitable for drupal_rewrite_settings().
    *
@@ -557,6 +564,7 @@ public function installDrupal() {
     $this->initSettings();
     $container = $this->initKernel(\Drupal::request());
     $this->initConfig($container);
+    $this->installThemeFromClassProperty($container);
     $this->installModulesFromClassProperty($container);
     $this->rebuildAll();
   }
diff --git a/core/tests/Drupal/Tests/Listeners/DeprecationListenerTrait.php b/core/tests/Drupal/Tests/Listeners/DeprecationListenerTrait.php
index 3c76756bd5..da5256c82f 100644
--- a/core/tests/Drupal/Tests/Listeners/DeprecationListenerTrait.php
+++ b/core/tests/Drupal/Tests/Listeners/DeprecationListenerTrait.php
@@ -140,6 +140,8 @@ public static function getSkippedDeprecations() {
       // This deprecation comes from behat/mink-browserkit-driver when updating
       // symfony/browser-kit to 4.3+.
       'The "Symfony\Component\BrowserKit\Response::getStatus()" method is deprecated since Symfony 4.3, use getStatusCode() instead.',
+      // @todo Remove in …
+      'Drupal\Tests\BrowserTestBase::$theme is required in drupal:9.0.0. See https://www.drupal.org/node/2352949',
     ];
   }