The problem is most likely due to:
HTML Code:
<base href="http://www.yesfans.com/forum.php/" /><!--[if IE]></base><![endif]-->
which is pulled from the URL settings of your forum.
All stylesheets and scripts use this base as well. So right now I see that stylesheets and scripts are loaded from
http://www.yesfans.com/forum.php/cli...0l/sidebar.css which doesn't exist.
http://www.yesfans.com/clientscript/...0l/sidebar.css does exist.