title: Adding class to sweet alert
body: <p>I am trying to add an extra class for my modal so I can select it from LESS and turn it's background to transparent. But customClass is not working. Is there any other way to do it. BTW I have already changed a lot with default classes so I need to do this for just one modal, can not effect the global swal.</p>
  
  <pre><code>   swal({
        title: success,
        showConfirmButton: false,
        html: true,
        customClass: ".swal-back"
     });
  </code></pre>