reverted:
--- b/README.txt
+++ a/README.txt
@@ -60,9 +60,6 @@
 that you can theme e-mails the same way you theme other content. The theme hook
 you need to use when interacting with the theming system is 'swiftmailer'.
 
-Add custom css to the template with the preprocess hook and convert them to
-inline-styles with the corresponding option under the message settings.
-
 2.1 Theme File
 
 The easiest way to theme e-mails is to create a file named
diff -u b/src/Form/MessagesForm.php b/src/Form/MessagesForm.php
--- b/src/Form/MessagesForm.php
+++ b/src/Form/MessagesForm.php
@@ -64,14 +64,6 @@
         as plain text as this is the content type Drupal by default will apply to all e-mails.'),
     );
 
-    $form['format']['css_inliner'] = array(
-      '#type' => 'checkbox',
-      '#title' => t('Convert CSS to inline styles'),
-      '#default_value' => $config->get('css_inliner'),
-      '#description' => t('The css of the rendered template (if set in template) will be parsed to inline styles 
-        to assure solid rendering in all email clients.'),
-    );
-
     $form['convert'] = array(
       '#type' => 'fieldset',
       '#title' => t('Plain Text Version'),
diff -u b/src/Plugin/Mail/SwiftMailer.php b/src/Plugin/Mail/SwiftMailer.php
--- b/src/Plugin/Mail/SwiftMailer.php
+++ b/src/Plugin/Mail/SwiftMailer.php
@@ -6,13 +6,16 @@
 use Drupal\Component\Utility\Html;
 use Drupal\Component\Utility\Unicode;
 use Drupal\Component\Utility\UrlHelper;
+use Drupal\Core\Asset\LibraryDiscoveryParser;
 use Drupal\Core\Config\ImmutableConfig;
 use Drupal\Core\Extension\ModuleHandlerInterface;
 use Drupal\Core\Mail\MailInterface;
+use Drupal\Core\Mail\MailManagerInterface;
 use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
 use Drupal\Core\Render\Markup;
 use Drupal\Core\Render\RendererInterface;
 use Drupal\Core\Site\Settings;
+use Drupal\mailsystem\MailsystemManager;
 use Drupal\swiftmailer\Utility\Conversion;
 use Exception;
 use Html2Text\Html2Text;
@@ -62,6 +65,16 @@
   protected $moduleHandler;
 
   /**
+   * @var \Drupal\Core\Mail\MailManagerInterface
+   */
+  protected $mailManager;
+
+  /**
+   * @var \Drupal\Core\Asset\LibraryDiscoveryParser
+   */
+  protected $libraryDiscoverParser;
+
+  /**
    * SwiftMailer constructor.
    *
    * @param \Drupal\Core\Config\ImmutableConfig $transport
@@ -69,13 +82,17 @@
    * @param \Psr\Log\LoggerInterface $logger
    * @param \Drupal\Core\Render\RendererInterface $renderer
    * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
+   * @param \Drupal\Core\Mail\MailManagerInterface $mail_manager
+   * @param \Drupal\Core\Asset\LibraryDiscoveryParser $library_discover_parser
    */
-  public function __construct(ImmutableConfig $transport, ImmutableConfig $message, LoggerInterface $logger, RendererInterface $renderer, ModuleHandlerInterface $module_handler) {
+  function __construct(ImmutableConfig $transport, ImmutableConfig $message, LoggerInterface $logger, RendererInterface $renderer, ModuleHandlerInterface $module_handler, MailManagerInterface $mail_manager, LibraryDiscoveryParser $library_discover_parser) {
     $this->config['transport'] = $transport->get();
     $this->config['message'] = $message->get();
     $this->logger = $logger;
     $this->renderer = $renderer;
     $this->moduleHandler = $module_handler;
+    $this->mailManager = $mail_manager;
+    $this->libraryDiscoverParser = $library_discover_parser;
   }
 
   /**
@@ -87,7 +104,9 @@
       $container->get('config.factory')->get('swiftmailer.message'),
       $container->get('logger.factory')->get('swiftmailer'),
       $container->get('renderer'),
-      $container->get('module_handler')
+      $container->get('module_handler'),
+      $container->get('plugin.manager.mail'),
+      $container->get('library.discovery.parser')
     );
   }
 
@@ -123,15 +142,28 @@
       }
 
       if ($this->config['message']['css_inliner']) {
-        $theme_path  =\Drupal::service('theme.manager')->getActiveTheme()->getPath();
-        $mail_css_path = realpath($theme_path . '/css/mail.css');
 
-        // Doesn't exist a mail.css in the default theme folder under the css folder.
-        if($mail_css_path) {
-          $message['body'] = (new CssToInlineStyles())->convert($message['body'], file_get_contents($mail_css_path));
-        } else {
-          // TODO Decide if it should show a non-existing file error or if the
-          // swiftmailer module should have a default mail.css file and use that one in this case.          
+        // Attempt to use the email theme defined in MailSystem.
+        if ($this->mailManager instanceof MailsystemManager) {
+          $theme_name = $this->mailManager->getMailTheme();
+        }
+        // Default to the active theme if MailsystemManager isn't used.
+        else {
+          $theme_name = $this->themeManager->getActiveTheme()->getName();
+        }
+
+        // Parse libraries defined by the theme.
+        $libraries = $this->libraryDiscoverParser->buildByExtension($theme_name);
+
+        // If the theme contains a mail library, inline each
+        // css file defined in the library.
+        if (!empty($libraries['mail']['css'])) {
+          foreach ($libraries['mail']['css'] as $library_css) {
+            $mail_css_path = realpath($library_css['data']);
+            if (file_exists($mail_css_path)) {
+              $message['body'] = (new CssToInlineStyles())->convert($message['body'],  file_get_contents($mail_css_path));
+            }
+          }
         }
       }
     }
only in patch2:
unchanged:
--- a/config/install/swiftmailer.message.yml
+++ b/config/install/swiftmailer.message.yml
@@ -1,4 +1,5 @@
 format: text/plain
 respect_format: true
+css_inliner: false
 convert_mode: false
 character_set: UTF-8
only in patch2:
unchanged:
--- a/config/schema/swiftmailer.schema.yml
+++ b/config/schema/swiftmailer.schema.yml
@@ -8,6 +8,9 @@ swiftmailer.message:
     respect_format:
       type: boolean
       label: 'Respect format'
+    css_inliner:
+      type: boolean
+      label: 'Inline CSS'
     convert_mode:
       type: boolean
       label: 'Respect format'