Knowing CSS helps a lot, and while xanga’s interface helps you customize your site more thoroughly than before, a little more customization can’t hurt, right? Once you get the basics down, they become second nature and maybe you could even move on to more advanced stuff, like div overlays. =]

I myself learned primarily though studying other people’s codes and picking out what codes had what function and such. Make sure that if you do this, though, that you don’t steal codes. Some sites have style sheets laid out that you can customize and study yourself if you want to learn!