Download here

Idea of plugin and design: Aryandhani

Instructions:

Copy preview.md in content folder.
Copy preview.html in theme folder.
Activate plugin in config.php

links example:

<a href="http://yoursite.com/preview?theme=monstra.org" title="example">Monstra.org</a>

Preview

Note:

If you write http://  like a preview?theme=http://site.com  this not working