This explains it pretty well, it's not actual CSS but like a BBCode version. A simple CSS code looks something like this:
textttttttttttttttttt=
textttttttttttttttttt
Basically, you just add codes in
like HTML kinda. Border radius gives curved borders, padding is the amount of space between the text and the border (each pixel amount for top, bottom, left, and right), color is the text color, and the rest is self-explanatory.