Is There a Way to Use CSS?

So now that I've got so visual is important for me, I care a lot about the visual stuff. So one thing that I'm now going to be doing and one of the things I'm looking at today is to start using CSS because I have that knowledge enough. Most of it's HTML but you can just use HTML and put something inside of the HTML and it will change the formatting. You could do it potentially in your theme, your CSS theme, but I'm not entirely sure whether that would work in live preview though.