It's slowly sinking in.... but

Hi guys.
So, I bought the book :)
...and I learned a thing or two. But it also left me a bit confused.
I read that HTML says WHAT to display, and CSS says HOW to display it. All very well and good.

But where do I define the look of the buttons for the comic nav?
I've look all over for a place to put my code to define their look, and even ended up pasted it everywhere I thought might make a difference.
It didn't.

Would you please shed some light.

Thanks