Honestly I've always found the best way to learn CSS is to take a pre-made code and edit it yourself to see what happens. For example, use this basic code [cd="height: 100px; width: 100px; background-color: red; padding: 5px; border-size: 5px; border: solid; border-color: black;"]HI THIS IS TEXT[/cd] and it becomes:
HI THIS IS TEXT
Then try changing around some of the values. For example I'll change 'height: 100px' to 'height: 30px' and 'background-color: red' to 'background-colour: blue' to get this:
HI THIS IS TEXT
A shorter box with a blue background. Or you can change the padding from 5px to 25 px to get this:
HI THIS IS TEXT
A box where the text starts further in from the borders (AKA padding).
And then just try editing some numbers or fields until you start to see what different things do. Then once you've gotten the basics of editing CSS down, take a look at a thread like
this to see other fields that you can add in, such as linking a background image for the CSS. And of course you've got a whole community here to help you if you get stuck so don't hesitate to ask if you need help understanding something. :)