I'm a newbie to css myself, so I'm not sure that I can give you the right answer, but yes, the color will be specified in your stylesheet.

/includes/templates/YourTemplate/css/stylesheet.css

It seems you have already managed it, but just put it in the wrong place.

I think that you have now colored #categories
Change that to #categoriesContent

That is going to color the background of the box containing your categories, but not the heading "Categories." I'm not sure if that is what you want.

It is also not going to color the rounded edge at the top or bottom. In order to color that you need to add your color style to
.xb1
.xb2
.xb3
and
.xb4

Sorry, in a rush. I'll try and get back to you about the heading later.

Audra