diff --git a/mailsystem.module b/mailsystem.module
index 701a263..2ce3685 100644
--- a/mailsystem.module
+++ b/mailsystem.module
@@ -6,14 +6,6 @@
  */
 
 /**
-* Implements hook_theme_registry_alter().
-*/
-function mailsystem_theme_registry_alter(&$theme_registry) {
-  module_load_include('inc', 'mailsystem', 'mailsystem.theme');
-  //return mailsystem_theme_theme_registry_alter($theme_registry);
-}
-
-/**
  * Retrieves the key of the theme used to render the emails.
  *
  * @todo Add some kind of hook to let other modules alter this behavior.
diff --git a/mailsystem.services.yml b/mailsystem.services.yml
new file mode 100644
index 0000000..3538b01
--- /dev/null
+++ b/mailsystem.services.yml
@@ -0,0 +1,8 @@
+services:
+  mailsystem.theme.registry:
+    class: Drupal\Core\Theme\Registry
+    arguments: ['@app.root', '@cache.default', '@lock', '@module_handler', '@theme_handler', '@theme.initialization']
+    tags:
+      - { name: needs_destruction }
+    calls:
+      - [setThemeManager, ['@theme.manager']]
diff --git a/mailsystem.theme.inc b/mailsystem.theme.inc
deleted file mode 100644
index ae16ab7..0000000
--- a/mailsystem.theme.inc
+++ /dev/null
@@ -1,84 +0,0 @@
-<?php
-
-/**
-* @file
-* The theme system, which controls the output of email messages.
-*/
-
-/**
-* Implements hook_theme_registry_alter().
-*/
-function mailsystem_theme_theme_registry_alter(&$theme_registry) {
-  $theme_key = \Drupal::theme()->getActiveTheme()->getName();
-  static $recursion_prevention = FALSE;
-
-  // Prevent recursive execution.
-  if ($recursion_prevention) {
-    return;
-  }
-  $recursion_prevention = TRUE;
-  $mailsystem_theme = mailsystem_get_mail_theme();
-
-  // Only take action if the mailsystem theme is not the current theme.
-  if ($mailsystem_theme != $theme_key) {
-    $theme_handler = \Drupal::service('theme_handler');
-    $themes = $theme_handler->listInfo();
-    // Get the mailsystem theme to be used for rendering emails.
-    if (isset($themes[$mailsystem_theme])) {
-      $theme = clone $themes[$mailsystem_theme];
-      if (isset($theme)) {
-        // Establish variables for further processing.
-        $base_theme = array();
-        if (isset($theme->base_themes)) {
-          foreach (array_keys($theme->base_themes) as $base) {
-            $base_theme[$base] = clone $themes[$base];
-          }
-        }
-        if (isset($theme->base_theme) && !isset($base_theme[$theme->base_theme])) {
-          $base_theme[$theme->base_theme] = clone $themes[$theme->base_theme];
-        }
-        if (isset($theme->engine)) {
-          $theme_engine = $theme->engine;
-        }
-
-        // Include template files to let _theme_load_registry add preprocess
-        // functions.
-        include_once(drupal_get_path('theme', $theme->name) . '/template.php');
-        foreach ($base_theme as $base) {
-          include_once(drupal_get_path('theme', $base->name) . '/template.php');
-        }
-
-        // Get the theme_registry cache.
-        // @todo replace with something like $cache = \Drupal::service('theme.registry')->get();
-        $cache = _theme_load_registry($theme, $base_theme, $theme_engine);
-
-        // Change the registry for hooks with a 'mail theme' element.
-        foreach ($theme_registry as $name => $hook) {
-          if (!empty($hook['mail theme'])) {
-            if (isset($cache[$name])) {
-              $cache[$name]['includes'][] = drupal_get_path('theme', $theme->name) . '/template.php';
-              foreach ($base_theme as $base) {
-                $cache[$name]['includes'][] = drupal_get_path('theme', $base->name) . '/template.php';
-              }
-              // Change the current registry for the new record.
-              $theme_registry[$name] = $cache[$name];
-            }
-
-            // Look for template suggestions.
-            foreach ($cache as $cache_name => $cache_hook) {
-              if (strpos($cache_name, $name . '__') !== FALSE) {
-                $cache_hook['includes'][] = drupal_get_path('theme', $theme->name) . '/template.php';
-                foreach ($base_theme as $base) {
-                  $cache_hook['includes'][] = drupal_get_path('theme', $base->name) . '/template.php';
-                }
-                // Change the current registry for the new record.
-                $theme_registry[$cache_name] = $cache_hook;
-              }
-            }
-          }
-        }
-      }
-    }
-  }
-  $recursion_prevention = FALSE;
-}
diff --git a/src/MailsystemManager.php b/src/MailsystemManager.php
index f337036..8f6e8e9 100644
--- a/src/MailsystemManager.php
+++ b/src/MailsystemManager.php
@@ -12,11 +12,13 @@ use Drupal\Component\Utility\SafeMarkup;
 use Drupal\Core\Cache\CacheBackendInterface;
 use Drupal\Core\Config\ConfigFactoryInterface;
 use Drupal\Core\Extension\ModuleHandlerInterface;
-use Drupal\Core\Language\LanguageManagerInterface;
 use Drupal\Core\Logger\LoggerChannelFactoryInterface;
 use Drupal\Core\Mail\MailInterface;
 use Drupal\Core\Mail\MailManager;
 use Drupal\Core\StringTranslation\TranslationInterface;
+use Drupal\Core\Theme\Registry;
+use Drupal\Core\Theme\ThemeManager;
+use Drupal\Core\Theme\ThemeManagerInterface;
 
 /**
  * Factory for creating mail system objects based on BasePlugin's.
@@ -38,11 +40,65 @@ class MailsystemManager extends MailManager {
   protected $mailsystemConfig;
 
   /**
+   * @var \Drupal\Core\Theme\ThemeManagerInterface
+   */
+  protected $themeManager;
+
+  /**
+   * @var \Drupal\Core\Theme\Registry
+   */
+  protected $defaultThemeRegistry;
+
+  /**
+   * @var \Drupal\Core\Theme\Registry
+   */
+  protected $mailThemeRegistry;
+
+  /**
    * {@inheritdoc}
    */
-  public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, ModuleHandlerInterface $module_handler, ConfigFactoryInterface $config_factory, LoggerChannelFactoryInterface $logger_factory, TranslationInterface $string_translation) {
+  public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, ModuleHandlerInterface $module_handler, ConfigFactoryInterface $config_factory, LoggerChannelFactoryInterface $logger_factory, TranslationInterface $string_translation, ThemeManagerInterface $theme_manager, Registry $default_theme_registry, Registry $mail_theme_registry) {
     parent::__construct($namespaces, $cache_backend, $module_handler, $config_factory, $logger_factory, $string_translation);
     $this->mailsystemConfig = $config_factory->get('mailsystem.settings');
+    $this->themeManager = $theme_manager;
+    $this->defaultThemeRegistry = $default_theme_registry;
+    $this->mailThemeRegistry = $mail_theme_registry;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function mail($module, $key, $to, $langcode, $params = array(), $reply = NULL, $send = TRUE) {
+    // Switch the theme to the configured mail theme.
+    $mail_theme = $this->getMailTheme();
+    $current_active_theme = $this->themeManager->getActiveTheme();
+    if ($mail_theme != $current_active_theme->getName()) {
+      $this->themeManager->setActiveTheme(\Drupal::service('theme.initialization')->initTheme($mail_theme));
+
+      // The theme registry returns the same registry object no matter which
+      // theme is currently active. This works around that by having a duplicate
+      // service, that is only called when the mail theme is acive.
+      // @todo: This will not work if this can not be called. Remove this once
+      //   https://www.drupal.org/node/2640962 is committed.
+      if ($this->themeManager instanceof ThemeManager) {
+        $this->themeManager->setThemeRegistry($this->mailThemeRegistry);
+      }
+    }
+
+    try {
+      $message = parent::mail($module, $key, $to, $langcode, $params, $reply, $send);
+    }
+    finally {
+      // Revert the active theme, this is done inside a finally block so it is
+      // executed even if an exception is thrown during sending a mail.
+      if ($mail_theme != $current_active_theme->getName()) {
+        $this->themeManager->setActiveTheme($current_active_theme);
+        if ($this->themeManager instanceof ThemeManager) {
+          $this->themeManager->setThemeRegistry($this->defaultThemeRegistry);
+        }
+      }
+    }
+    return $message;
   }
 
   /**
@@ -53,8 +109,8 @@ class MailsystemManager extends MailManager {
     $key = isset($options['key']) ? $options['key'] : '';
 
     return new Adapter(
-      $this->getPluginInstance($module, $key, self::MAILSYSTEM_TYPE_FORMATTING),
-      $this->getPluginInstance($module, $key, self::MAILSYSTEM_TYPE_SENDING)
+      $this->getPluginInstance($module, $key, static::MAILSYSTEM_TYPE_FORMATTING),
+      $this->getPluginInstance($module, $key, static::MAILSYSTEM_TYPE_SENDING)
     );
   }
 
@@ -111,4 +167,31 @@ class MailsystemManager extends MailManager {
     return $this->instances[$plugin_id];
   }
 
+  /**
+   * Retrieves the key of the theme used to render the emails.
+   */
+  public function getMailTheme() {
+    $theme = $this->mailsystemConfig->get('theme');
+    switch ($theme) {
+      case 'default':
+        $theme = $this->configFactory->get('system.theme')->get('default');
+        break;
+      case 'current':
+        $theme = $this->themeManager->getActiveTheme()->getName();
+        break;
+      case 'domain':
+        // Fetch the theme for the current domain.
+        // @todo: Reimplement this as soon as module port or similar module is around.
+        if (FALSE && \Drupal::moduleHandler()->moduleExists('domain_theme')) {
+          // Assign the selected theme, based on the active domain.
+          global $_domain;
+          $domain_theme = domain_theme_lookup($_domain['domain_id']);
+          // The above returns -1 on failure.
+          $theme = ($domain_theme != -1) ? $domain_theme['theme'] : $this->themeManager->getActiveTheme()->getName();
+        }
+        break;
+    }
+    return $theme;
+  }
+
 }
diff --git a/src/MailsystemServiceProvider.php b/src/MailsystemServiceProvider.php
index 7c615da..e46c598 100644
--- a/src/MailsystemServiceProvider.php
+++ b/src/MailsystemServiceProvider.php
@@ -29,7 +29,11 @@ class MailsystemServiceProvider implements ServiceProviderInterface, ServiceModi
   public function alter(ContainerBuilder $container) {
     // Overrides mail-factory class to use our own mail manager.
     $container->getDefinition('plugin.manager.mail')
-      ->setClass('Drupal\mailsystem\MailsystemManager');
+      ->setClass('Drupal\mailsystem\MailsystemManager')
+      ->addArgument(new Reference('theme.manager'))
+      ->addArgument(new Reference('theme.registry'))
+      ->addArgument(new Reference('mailsystem.theme.registry'))
+      ->addArgument(new Reference('theme.initialization'));
   }
 
 }
diff --git a/src/Tests/MailsystemTestThemeTest.php b/src/Tests/MailsystemTestThemeTest.php
new file mode 100644
index 0000000..28da490
--- /dev/null
+++ b/src/Tests/MailsystemTestThemeTest.php
@@ -0,0 +1,77 @@
+<?php
+/**
+ * @file
+ * Contains \Drupal\mailsystem\Tests\MailsystemTestThemeTest.
+ */
+
+namespace Drupal\mailsystem\Tests;
+
+use Drupal\simpletest\WebTestBase;
+
+/**
+ * Tests mail theme for formatting emails using a theme template.
+ *
+ * @group mailsystem
+ */
+class MailsystemTestThemeTest extends WebTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  public static $modules = [
+    'mailsystem',
+    'mailsystem_test',
+  ];
+
+  /**
+   * The Mailsystem settings config object.
+   *
+   * @var \Drupal\Core\Config\Config
+   */
+  protected $config;
+
+  public function setUp() {
+    parent::setUp();
+    $this->config = $this->config('mailsystem.settings');
+
+
+  }
+
+  /**
+   * Tests the mailsystem theme.
+   */
+  public function testMailsystemTheme() {
+
+    // Manually configure the test mail collector implementation to prevent
+    // tests from sending out emails and collect them in state instead.
+    // While this should be enforced via settings.php prior to installation,
+    // some tests expect to be able to test mail system implementations.
+    $this->config
+      ->set('defaults.sender', 'test_mail_collector')
+      ->set('defaults.formatter', 'test_mail_collector')
+      ->save();
+
+    // Send an email.
+    $this->drupalGet('/mailsystem-test/theme');
+    $mails = $this->drupalGetMails();
+
+    // Check the configuration and if the correct theme was used in mails.
+    $this->assertEqual($this->config->get('theme'), 'current');
+    $this->assertTrue(strpos($mails[0]['body'], 'Anonymous (not verified)') !== FALSE);
+
+    // Install the test theme.
+    \Drupal::service('theme_handler')->install(array('mailsystem_test_theme'));
+    $this->config->set('theme', 'mailsystem_test_theme')->save();
+
+    // Send another email.
+    $this->drupalGet('/mailsystem-test/theme');
+    $mails = $this->drupalGetMails();
+
+    // Check the new configuration and if the correct theme was used in mails.
+    $this->assertEqual($this->config->get('theme'), 'mailsystem_test_theme');
+    $this->assertTrue(strpos($mails[1]['body'], 'Mailsystem test theme') !== FALSE);
+  }
+
+}
diff --git a/tests/modules/mailsystem_test/mailsystem_test.info.yml b/tests/modules/mailsystem_test/mailsystem_test.info.yml
new file mode 100644
index 0000000..c8626be
--- /dev/null
+++ b/tests/modules/mailsystem_test/mailsystem_test.info.yml
@@ -0,0 +1,7 @@
+name: Mailsystem Test
+type: module
+package: Mail
+hidden: true
+core: 8.x
+dependencies:
+  - mailsystem
diff --git a/tests/modules/mailsystem_test/mailsystem_test.module b/tests/modules/mailsystem_test/mailsystem_test.module
new file mode 100644
index 0000000..2da1ed6
--- /dev/null
+++ b/tests/modules/mailsystem_test/mailsystem_test.module
@@ -0,0 +1,25 @@
+<?php
+
+/**
+ * @file
+ * Enables the use of personal and site-wide contact forms.
+ */
+
+use Drupal\user\Entity\User;
+
+/**
+ * Implements hook_mail().
+ */
+function mailsystem_test_mail($key, &$message, $params) {
+  switch ($key) {
+    case 'theme_test':
+      $account = User::load(\Drupal::currentUser()->id());
+      $username = array('#theme' => 'username', '#account' => $account);
+
+      $message['subject'] = t('Testing mail theme.');
+      $message['body'][] = (string) \Drupal::service('renderer')->renderPlain($username);
+      break;
+  }
+}
+
+
diff --git a/tests/modules/mailsystem_test/mailsystem_test.routing.yml b/tests/modules/mailsystem_test/mailsystem_test.routing.yml
new file mode 100644
index 0000000..fe2a6de
--- /dev/null
+++ b/tests/modules/mailsystem_test/mailsystem_test.routing.yml
@@ -0,0 +1,9 @@
+mailsystem_test.theme:
+  path: '/mailsystem-test/theme'
+  defaults:
+    _title: 'Mailsystem theme'
+    _controller: '\Drupal\mailsystem_test\Controller\MailsystemTestController::sendMail'
+  options:
+    no_cache: 'TRUE'
+  requirements:
+    _access: 'TRUE'
diff --git a/tests/modules/mailsystem_test/src/Controller/MailsystemTestController.php b/tests/modules/mailsystem_test/src/Controller/MailsystemTestController.php
new file mode 100644
index 0000000..ed3d49f
--- /dev/null
+++ b/tests/modules/mailsystem_test/src/Controller/MailsystemTestController.php
@@ -0,0 +1,28 @@
+<?php
+/**
+ * @file
+ * Contains \Drupal\mailsystem_test\Controller\MailsystemTestController.
+ */
+
+namespace Drupal\mailsystem_test\Controller;
+use Symfony\Component\HttpFoundation\Response;
+
+/**
+ * A mailsystem test controller for use by tests in this file.
+ */
+class MailsystemTestController {
+
+  /**
+   * Composes and optionally sends an email message.
+   *
+   * @return \Symfony\Component\HttpFoundation\RedirectResponse
+   */
+  public function sendMail() {
+    $module = 'mailsystem_test';
+    $key = 'theme_test';
+    $to = 'theme_test@example.com';
+    $langcode = \Drupal::languageManager()->getDefaultLanguage()->getId();
+    \Drupal::service('plugin.manager.mail')->mail($module, $key, $to, $langcode);
+    return new Response('', 204);
+  }
+}
diff --git a/tests/src/Unit/MailsystemManagerTest.php b/tests/src/Unit/MailsystemManagerTest.php
index fffe9a2..ac4435a 100644
--- a/tests/src/Unit/MailsystemManagerTest.php
+++ b/tests/src/Unit/MailsystemManagerTest.php
@@ -6,6 +6,8 @@
 
 namespace Drupal\Tests\mailsystem\Unit;
 
+use Drupal\Core\Theme\Registry;
+use Drupal\Core\Theme\ThemeManager;
 use Drupal\mailsystem\MailsystemManager;
 use Drupal\Tests\UnitTestCase;
 
@@ -60,7 +62,11 @@ class MailsystemManagerTest extends UnitTestCase {
     $logger_factory = $this->getMock('Drupal\Core\Logger\LoggerChannelFactoryInterface');
     $string_translation = $this->getMock('Drupal\Core\StringTranslation\TranslationInterface');
 
-    $this->mailManager = new MailsystemManager($this->getMock('\Traversable'), $this->getMock('\Drupal\Core\Cache\CacheBackendInterface'), $this->getMock('\Drupal\Core\Extension\ModuleHandlerInterface'), $this->configFactory, $logger_factory, $string_translation);
+    $theme_manager = $this->prophesize(ThemeManager::class);
+    $default_theme_registry = $this->prophesize(Registry::class);
+    $mail_theme_registry = $this->prophesize(Registry::class);
+
+    $this->mailManager = new MailsystemManager($this->getMock('\Traversable'), $this->getMock('\Drupal\Core\Cache\CacheBackendInterface'), $this->getMock('\Drupal\Core\Extension\ModuleHandlerInterface'), $this->configFactory, $logger_factory, $string_translation, $theme_manager->reveal(), $default_theme_registry->reveal(), $mail_theme_registry->reveal());
   }
 
   public function testGetInstances_Default() {
diff --git a/tests/themes/mailsystem_test_theme/mailsystem_test_theme.info.yml b/tests/themes/mailsystem_test_theme/mailsystem_test_theme.info.yml
new file mode 100644
index 0000000..e2c9ae4
--- /dev/null
+++ b/tests/themes/mailsystem_test_theme/mailsystem_test_theme.info.yml
@@ -0,0 +1,16 @@
+name: 'Mailsystem test theme'
+type: theme
+description: 'Theme for testing mailsystem'
+version: VERSION
+core: 8.x
+regions:
+  sidebar_first: 'Left sidebar'
+  sidebar_second: 'Right sidebar'
+  content: Content
+  header: Header
+  footer: Footer
+  highlighted: Highlighted
+  help: Help
+regions_hidden:
+  - sidebar_first
+  - sidebar_second
diff --git a/tests/themes/mailsystem_test_theme/templates/username.html.twig b/tests/themes/mailsystem_test_theme/templates/username.html.twig
new file mode 100644
index 0000000..43c081c
--- /dev/null
+++ b/tests/themes/mailsystem_test_theme/templates/username.html.twig
@@ -0,0 +1,26 @@
+{#
+/**
+ * @file
+ * Default theme implementation for displaying a username in a mail.
+ *
+ * Available variables:
+ * - account: The full account information for the user.
+ * - name: The user's name, sanitized.
+ * - extra: Additional text to append to the user's name, sanitized.
+ * - link_path: The path or URL of the user's profile page, home page,
+ *   or other desired page to link to for more information about the user.
+ * - link_options: Options to pass to the url() function's $options parameter if
+ *   linking the user's name to the user's page.
+ * - attributes: HTML attributes for the containing element.
+ *
+ * @see template_preprocess_username()
+ *
+ * @ingroup themeable
+ */
+#}
+<p>Mailsystem test theme</p>
+{% if link_path -%}
+  <a{{ attributes }}>{{ name }}{{ extra }}</a>
+{%- else -%}
+  <span{{ attributes }}>{{ name }}{{ extra }}</span>
+{%- endif -%}