Board Thread:Support Requests - Designing Your Wiki/@comment-5466786-20130724030947/@comment-4674838-20130724041413

If you mean how to apply CSS to your wiki, see Help:JavaScript and CSS Cheatsheet. If you know how to apply existing CSS codes to a wiki, you could at least copy/paste code from other people. (This is what the vast majority of people do.)

If you mean "what is CSS / how does it work / how do you write your own CSS code" – that's a much broader question. If you want to learn more about CSS, I would strongly recommend taking a web design class in school if that is available to you, or buying a textbook from a bookstore. It's not something you can learn from a simple explanation – you really have to explore it on your own and put in months of time/effort/work to learn it.