fix rendering of notifications with config custom theme

src/Illuminate/Mail/Markdown.php

@@ -174,4 +174,14 @@ public function theme($theme)
 
         return $this;
     }
+
+    /**
+     * Get the theme currently being used by the renderer.
+     *
+     * @return string
+     */
+    public function getTheme()
+    {
+        return $this->theme;
+    }
 }

src/Illuminate/Notifications/Messages/MailMessage.php

@@ -312,10 +312,10 @@ public function render()
             );
         }
 
-        return Container::getInstance()
-            ->make(Markdown::class)
-            ->theme($this->theme ?: 'default')
-            ->render($this->markdown, $this->data());
+        $markdown = Container::getInstance()->make(Markdown::class);
+
+        return $markdown->theme($this->theme ?: $markdown->getTheme())
+                ->render($this->markdown, $this->data());
     }
 
     /**