.htaccess
RewriteRule ^admin(/?)+$ admin.php [L]
или только с /
RewriteRule ^admin/$ admin.php [L]

в
/engine/skins/default.skin.php

<link href="engine/skins/stylesheets/application.css" rel="stylesheet" type="text/css" />

на

<link href="/engine/skins/stylesheets/application.css" rel="stylesheet" type="text/css" />

и поиск
engine/skins/images
на
/engine/skins/images