Tutorial: Creating a custom skin for MediaWiki 1.18+
blog.redwerks.org/2012/02/08/mediawiki-skinning-tutorial
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
This tutorial details the creation of a custom skin for MediaWiki. The skin is packaged up into a directory you can drop into a MediaWiki installation and require_once() to install just as any other extension.