implement theme fix

Author: taylorotwell

src/Illuminate/Mail/Mailable.php

@@ -2,6 +2,7 @@
 
 namespace Illuminate\Mail;
 
+use Illuminate\Config\Repository as ConfigRepository;
 use Illuminate\Container\Container;
 use Illuminate\Contracts\Filesystem\Factory as FilesystemFactory;
 use Illuminate\Contracts\Mail\Attachable;
@@ -325,7 +326,12 @@ protected function buildMarkdownView()
         $markdown = Container::getInstance()->make(Markdown::class);
 
         if (isset($this->theme)) {
-            $markdown->theme($this->theme);
+            $markdown->theme(
+                $this->theme,
+                Container::getInstance()
+                    ->get(ConfigRepository::class)
+                    ->get('mail.markdown.theme', 'default')
+            );
         }
 
         $data = $this->buildViewData();