Complex sentence: Wikipedia, as a website powered by MediaWiki, is a skinnable website, which means the presentational style of the pages can be changed, provided you have a (see below to change skin without logging in).
Split into simpler sentences: Wikipedia, as a website powered by MediaWiki, is a "skinnable" website, which you are able to change the presentational style of the pages provided. You have a (see below to change skin without logging in).