Skin:Empty

The Empty skin allows you to customize the look and feel of MediaWiki is as easy as edit wiki page.

While this is only a concept, but when relevant elements will be ready, you will be able to set a wiki page as a skin and to provide an opportunity for displayed page to interact with the skin so much as you want.

Perhaps this is exactly what is needed for third-party websites that use MediaWiki core but do not want to look like Wikipedia.