Issue #3417823 by FMB: Allow to dynamically add CSS

src/Plugin/Mail/SymfonyMailer.php

@@ -653,6 +653,11 @@ class SymfonyMailer implements MailInterface, ContainerFactoryPluginInterface {
         }
       }
 
+      // Let modules add CSS when calling MailManager::mail().
+      if (!empty($message['params']['css']) && is_string($message['params']['css'])) {
+        $css .= $message['params']['css'];
+      }
+
       if ($css) {
         $message['body'] = $this->cssInliner->convert($message['body'], $css);
       }