diff -u b/core/includes/install.core.inc b/core/includes/install.core.inc
--- b/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -482,8 +482,7 @@
       ->addArgument(new Reference('config.factory'))
       ->addArgument(new Reference('module_handler'))
       ->addArgument(new Reference('cache.cache'))
-      ->addArgument(new Reference('info_parser'))
-    ;
+      ->addArgument(new Reference('info_parser'));
   }
 
   // Set the request in the kernel to the new created Request above
diff -u b/core/lib/Drupal/Core/Theme/ThemeHandler.php b/core/lib/Drupal/Core/Theme/ThemeHandler.php
--- b/core/lib/Drupal/Core/Theme/ThemeHandler.php
+++ b/core/lib/Drupal/Core/Theme/ThemeHandler.php
@@ -7,10 +7,13 @@
 
 namespace Drupal\Core\Theme;
 
+use Drupal\Component\Utility\String;
 use Drupal\Core\Cache\CacheBackendInterface;
 use Drupal\Core\Config\ConfigFactory;
+use Drupal\Core\Extension\ExtensionNameLengthException;
 use Drupal\Core\Extension\ModuleHandlerInterface;
 use Drupal\Core\Routing\RouteBuilder;
+use Drupal\Core\SystemListing;
 use Drupal\Core\SystemListingInfo;
 use Drupal\Core\Extension\InfoParserInterface;
 
@@ -33,25 +36,82 @@
 
   protected $list = array();
 
-  public function __construct(ConfigFactory $config_factory, ModuleHandlerInterface $module_handler, CacheBackendInterface $cache_backend, InfoParserInterface $info_parser, RouteBuilder $route_builder = NULL) {
+  /**
+   * The config factory to get the enabled themes.
+   *
+   * @var \Drupal\Core\Config\ConfigFactory
+   */
+  protected $configFactory;
+
+  /**
+   * The module handler to fire themes_enabled/themes_disabled hooks.
+   *
+   * @var \Drupal\Core\Extension\ModuleHandlerInterface
+   */
+  protected $moduleHandler;
+
+  /**
+   * The cache backend to clear the local tasks cache.
+   *
+   * @var \Drupal\Core\Cache\CacheBackendInterface
+   */
+  protected $cacheBackend;
+
+  /**
+   * The info parser to parse the theme.info.yml files.
+   *
+   * @var \Drupal\Core\Extension\InfoParserInterface
+   */
+  protected $infoParser;
+
+  /**
+   * The route builder to rebuild the routes if a theme is enabled.
+   *
+   * @var \Drupal\Core\Routing\RouteBuilder
+   */
+  protected $routerBuilder;
+
+  /**
+   * The system listing info
+   *
+   * @var \Drupal\Core\SystemListingInfo
+   */
+  protected $systemListingInfo;
+
+  /**
+   * Constructs a new ThemeHandler.
+   *
+   * @param \Drupal\Core\Config\ConfigFactory $config_factory
+   *   The config factory to get the enabled themes.
+   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
+   *   The module handler to fire themes_enabled/themes_disabled hooks.
+   * @param \Drupal\Core\Cache\CacheBackendInterface $cache_backend
+   *   The cache backend to clear the local tasks cache.
+   * @param \Drupal\Core\Extension\InfoParserInterface $info_parser
+   *   The info parser to parse the theme.info.yml files.
+   * @param \Drupal\Core\Routing\RouteBuilder $route_builder
+   *   The route builder to rebuild the routes if a theme is enabled.
+   */
+  public function __construct(ConfigFactory $config_factory, ModuleHandlerInterface $module_handler, CacheBackendInterface $cache_backend, InfoParserInterface $info_parser, RouteBuilder $route_builder = NULL, SystemListingInfo $system_list_info = NULL) {
     $this->configFactory = $config_factory;
     $this->moduleHandler = $module_handler;
     $this->cacheBackend = $cache_backend;
     $this->infoParser = $info_parser;
     $this->routeBuilder = $route_builder;
+    $this->systemListingInfo = $system_list_info;
   }
 
   /**
    * {@inheritdoc}
    */
   public function enable(array $theme_list) {
-    drupal_clear_css_cache();
+    $this->clearCssCache();
     $theme_config = $this->configFactory->get('system.theme');
     $disabled_themes = $this->configFactory->get('system.theme.disabled');
     foreach ($theme_list as $key) {
       // Throw an exception if the theme name is too long.
       if (strlen($key) > DRUPAL_EXTENSION_NAME_MAX_LENGTH) {
-        throw new ExtensionNameLengthException(format_string('Theme name %name is over the maximum allowed length of @max characters.', array(
+        throw new ExtensionNameLengthException(String::format('Theme name %name is over the maximum allowed length of @max characters.', array(
           '%name' => $key,
           '@max' => DRUPAL_EXTENSION_NAME_MAX_LENGTH,
         )));
@@ -116,7 +176,7 @@
       // Also check that the site is not in the middle of an install or update.
       if (!defined('MAINTENANCE_MODE')) {
         try {
-          $themes = system_list('theme');
+          $themes = $this->systemThemeList();
         }
         catch (\Exception $e) {
           // If the database is not available, rebuild the theme data.
@@ -166,7 +226,7 @@
    */
   public function rebuildThemeData() {
     // Find themes.
-    $listing = new SystemListingInfo();
+    $listing = $this->getSystemListingInfo();
     $themes = $listing->scan('/^' . DRUPAL_PHP_FUNCTION_PATTERN . '\.info.yml$/', 'themes', 'name', 1);
     // Allow modules to add further themes.
     if ($module_themes = $this->moduleHandler->invokeAll('system_theme_info')) {
@@ -181,7 +241,7 @@
     }
 
     // Find theme engines
-    $listing = new SystemListingInfo();
+    $listing = $this->getSystemListingInfo();
     $engines = $listing->scan('/^' . DRUPAL_PHP_FUNCTION_PATTERN . '\.engine$/', 'themes/engines', 'name', 1);
 
     // Set defaults for theme info.
@@ -225,7 +285,7 @@
       // Invoke hook_system_info_alter() to give installed modules a chance to
       // modify the data in the .info.yml files if necessary.
       $type = 'theme';
-      drupal_alter('system_info', $themes[$key]->info, $themes[$key], $type);
+      $this->moduleHandler->alter('system_info', $themes[$key]->info, $themes[$key], $type);
 
       if (!empty($themes[$key]->info['base theme'])) {
         $sub_themes[] = $key;
@@ -348,6 +408,13 @@
     return $current_base_theme;
   }
 
+  protected function getSystemListingInfo() {
+    if (!isset($this->systemListingInfo)) {
+      $this->systemListingInfo = new SystemListing();
+    }
+    return $this->systemListingInfo;
+  }
+
   /**
    * Installs the default theme config.
    *
@@ -362,6 +429,8 @@
       $this->routeBuilder->rebuild();
     }
 
+    // @todo It feels wrong to have the requirement to clear the local tasks
+    //   cache here.
     $this->cacheBackend->deleteTags(array('local_task' => 1));
     $this->themeRegistryRebuild();
   }
@@ -377 +446,5 @@
-} 
+  protected function systemThemeList() {
+    return system_list('theme');
+  }
+
+}
diff -u b/core/modules/system/system.module b/core/modules/system/system.module
--- b/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -2161,6 +2161,10 @@
   if (drupal_container()->isScopeActive('request')) {
     $request = \Drupal::request();
     $path = $request->attributes->get('_system_path');
+    debug($path);
+    debug(path_is_admin($path));
+    debug(user_access('view the administration theme'));
+    debug(\Drupal::config('system.theme')->get('admin'));
     if (user_access('view the administration theme') && path_is_admin($path)) {
       return \Drupal::config('system.theme')->get('admin');
     }
only in patch2:
unchanged:
--- a/core/lib/Drupal/Core/DependencyInjection/UpdateServiceProvider.php
+++ b/core/lib/Drupal/Core/DependencyInjection/UpdateServiceProvider.php
@@ -9,6 +9,7 @@
 
 use Drupal\Core\DependencyInjection\ContainerBuilder;
 use Drupal\Core\DependencyInjection\ServiceProviderInterface;
+use Symfony\Component\DependencyInjection\Reference;
 
 /**
  * ServiceProvider class for update.php service overrides.
@@ -38,6 +39,12 @@ public function register(ContainerBuilder $container) {
       ->register("cache_factory", 'Drupal\Core\Cache\MemoryBackendFactory');
     $container
       ->register('router.builder', 'Drupal\Core\Routing\RouteBuilderStatic');
+
+    $container->register('theme_handler', 'Drupal\Core\Theme\ThemeHandler')
+      ->addArgument(new Reference('config.factory'))
+      ->addArgument(new Reference('module_handler'))
+      ->addArgument(new Reference('cache.cache'))
+      ->addArgument(new Reference('info_parser'));
   }
 
 }
only in patch2:
unchanged:
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Theme/ThemeHandlerTest.php
@@ -0,0 +1,261 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Tests\Core\Theme\ThemeHandlerTest.
+ */
+
+namespace Drupal\Tests\Core\Theme;
+
+use Drupal\Core\Extension\InfoParser;
+use Drupal\Core\Theme\ThemeHandler;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * Tests the theme handler.
+ *
+ * @group Drupal
+ * @group Theme
+ *
+ * @see \Drupal\Core\Theme\ThemeHandler
+ */
+class ThemeHandlerTest extends UnitTestCase {
+
+  /**
+   * The mocked route builder.
+   *
+   * @var \Drupal\Core\Routing\RouteBuilder|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $routeBuilder;
+
+  /**
+   * The mocked info parser.
+   *
+   * @var \Drupal\Core\Extension\InfoParserInterface|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $infoParser;
+
+  /**
+   * The mocked cache backend.
+   *
+   * @var \Drupal\Core\Cache\CacheBackendInterface|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $cacheBackend;
+
+  /**
+   * The mocked config factory.
+   *
+   * @var \Drupal\Core\Config\ConfigFactory|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $configFactory;
+
+  /**
+   * The mocked module handler.
+   *
+   * @var \Drupal\Core\Extension\ModuleHandlerInterface|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $moduleHandler;
+
+  /**
+   * The system listing info.
+   *
+   * @var \Drupal\Core\SystemListingInfo|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $systemListingInfo;
+
+  /**
+   * The tested theme handler.
+   *
+   * @var \Drupal\Core\Theme\ThemeHandler|\Drupal\Tests\Core\Theme\TestThemeHandler
+   */
+  protected $themeHandler;
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getInfo() {
+    return array(
+      'name' => 'Theme handler',
+      'description' => 'Tests the theme handler.',
+      'group' => 'Theme',
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    $this->configFactory = $this->getConfigFactoryStub(array('system.theme' => array(), 'system.theme.disabled' => array()));
+    $this->moduleHandler = $this->getMock('Drupal\Core\Extension\ModuleHandlerInterface');
+    $this->cacheBackend = $this->getMock('Drupal\Core\Cache\CacheBackendInterface');
+    $this->infoParser = $this->getMock('Drupal\Core\Extension\InfoParserInterface');
+    $this->routeBuilder = $this->getMockBuilder('Drupal\Core\Routing\RouteBuilder')
+      ->disableOriginalConstructor()
+      ->getMock();
+    $this->systemListingInfo = $this->getMockBuilder('Drupal\Core\SystemListingInfo')
+      ->disableOriginalConstructor()
+      ->getMock();
+
+    $this->themeHandler = new TestThemeHandler($this->configFactory, $this->moduleHandler, $this->cacheBackend, $this->infoParser, $this->routeBuilder, $this->systemListingInfo);
+  }
+
+  /**
+   * Tests enabling a theme with a name longer than 50 chars.
+   *
+   * @expectedException \Drupal\Core\Extension\ExtensionNameLengthException
+   * @expectedExceptionMessage Theme name <em class="placeholder">thisNameIsFarTooLong0000000000000000000000000000051</em> is over the maximum allowed length of 50 characters.
+   */
+  public function testThemeEnableWithTooLongName() {
+    $this->themeHandler->enable(array('thisNameIsFarTooLong0000000000000000000000000000051'));
+  }
+
+  /**
+   * Tests enabling a single theme.
+   *
+   * @see \Drupal\Core\Theme\ThemeHandler::enable()
+   */
+  public function testEnableSingleTheme() {
+    $theme_list = array('theme_test');
+
+    $this->configFactory->get('system.theme')
+      ->expects($this->once())
+      ->method('set')
+      ->with('enabled.theme_test', 0)
+      ->will($this->returnSelf());
+    $this->configFactory->get('system.theme')
+      ->expects($this->once())
+      ->method('save');
+
+    $this->configFactory->get('system.theme.disabled')
+      ->expects($this->once())
+      ->method('clear')
+      ->with('theme_test')
+      ->will($this->returnSelf());
+    $this->configFactory->get('system.theme.disabled')
+      ->expects($this->once())
+      ->method('save');
+
+    // Ensure that the themes_enabled hook is fired.
+    $this->moduleHandler->expects($this->once())
+      ->method('invokeAll')
+      ->with('themes_enabled', array($theme_list));
+
+    $this->themeHandler->enable($theme_list);
+
+    $this->assertTrue($this->themeHandler->clearedCssCache);
+    $this->assertTrue($this->themeHandler->registryRebuild);
+    $this->assertTrue($this->themeHandler->installedDefaultConfig['theme_test']);
+  }
+
+  /**
+   * Ensures that enabling a theme does clear the theme info listing.
+   *
+   * @see \Drupal\Core\Theme\ThemeHandler::listInfo()
+   */
+  public function testEnableAndListInfo() {
+    $this->configFactory->get('system.theme')
+      ->expects($this->exactly(2))
+      ->method('set')
+      ->will($this->returnSelf());
+
+    $this->configFactory->get('system.theme.disabled')
+      ->expects($this->exactly(2))
+      ->method('clear')
+      ->will($this->returnSelf());
+
+    $this->themeHandler->enable(array('bartik'));
+    $this->themeHandler->systemList['bartik'] = (object) array(
+      'name' => 'bartik',
+      'info' => array(
+        'stylesheets' => array(
+          'all' => array(
+            'css/layout.css',
+            'css/style.css',
+            'css/colors.css',
+          ),
+        ),
+        'scripts' => array(
+          'example' => 'theme.js',
+        ),
+        'engine' => 'twig',
+        'base theme' => 'stark',
+      ),
+    );
+
+    $list_info = $this->themeHandler->listInfo();
+    $this->assertCount(1, $list_info);
+
+    $this->assertEquals($this->themeHandler->systemList['bartik']->info['stylesheets'], $list_info['bartik']->stylesheets);
+    $this->assertEquals($this->themeHandler->systemList['bartik']->scripts, $list_info['bartik']->scripts);
+    $this->assertEquals('twig', $list_info['bartik']->engine);
+    $this->assertEquals('stark', $list_info['bartik']->base_theme);
+    $this->assertEquals(0, $list_info['bartik']->status);
+
+    $this->themeHandler->systemList['seven'] = (object) array(
+      'name' => 'seven',
+      'info' => array(
+        'stylesheets' => array(
+          'screen' => array(
+            'style.css',
+          ),
+        ),
+        'scripts' => array(),
+      ),
+      'status' => 1,
+    );
+
+    $this->themeHandler->enable(array('seven'));
+
+    $list_info = $this->themeHandler->listInfo();
+    $this->assertCount(2, $list_info);
+
+    $this->assertEquals($this->themeHandler->systemList['seven']->info['stylesheets'], $list_info['seven']->stylesheets);
+    $this->assertEquals(1, $list_info['seven']->status);
+  }
+
+}
+
+/**
+ * Extends the default theme handler to mock some drupal_ methods.
+ */
+class TestThemeHandler extends ThemeHandler {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function clearCssCache() {
+    $this->clearedCssCache = TRUE;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function themeRegistryRebuild() {
+    $this->registryRebuild = TRUE;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function configInstallDefaultConfig($theme) {
+    $this->installedDefaultConfig[$theme] = TRUE;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function systemThemeList() {
+    return $this->systemList;
+  }
+
+}
+
+if (!defined('DRUPAL_EXTENSION_NAME_MAX_LENGTH')) {
+  define('DRUPAL_EXTENSION_NAME_MAX_LENGTH', 50);
+}
+if (!defined('DRUPAL_PHP_FUNCTION_PATTERN')) {
+  define('DRUPAL_PHP_FUNCTION_PATTERN', '[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*');
+}
+if (!defined('DRUPAL_ROOT')) {
+  define('DRUPAL_ROOT', dirname(dirname(substr(__DIR__, 0, -strlen(__NAMESPACE__)))));
+}