site/skins folder
Place your custom skins here.
A skin is always a PHP file, that applies the output of wiki into a HTML template, and at a least one CSS file that applies the style.
The code defines the PHP file. The CSS file is included by the PHP file directly.
To write your custom skins, get inspired by the skins in the inc/skins folder.
Include your skins in the site/configuration.php file.