Go to Settings -> Design.
Scroll to the bottom and you find a button that says "Edit CSS". In there you'll find two boxes where you can copy in code snippets.
Please note that we do not support custom css, if you don't know how to fix problems with it don't use a lot of it.
With that said, if you have a small change you want do make, ask us maybe we have that code-snippet laying around and can put it in for you.