I am a Wikipedia editor, and I find that I can adjust the appearance of what I see by applying CSS in Preferences / Appearance.
For the fun of it I have set up a MediaWiki site on my shared hosted site, and I find that I don't have the same options in Preferences / Appearance. To be specific, on Wikipedia I see that I can do various things with regard to the Vector skin:
I can choose Custom CSS and add "font-size: 1.1em;" to the Body tag and make things a bit larger for my older eyes. However, in my installation my skins are like this:
Vector (default | Preview)
I have only just set this site up a few days ago, and there's no content to speak of yet, and I have yet to learn how to do all the things I need to know, but I was surprised I couldn't set custom CSS! Is there some setting somewhere that I, as the Boss, can make custom CSS available?