Not all class and id names have rules in the default stylesheet, so if you need to style one of those elements, you need to add a rule in the stylesheet for that class or id name.

Personally, I wouldn't bother trying to figure out mods like Categories Dressing until you have the basics figured out.

I find the Web Developer plugin for Firefox to be more useful and easier to use than the tools built in to Chrome or Internet Explorer.

Mostly it's just a matter of trying stuff. Most of what I know about Zencart styling I learned trying to help other Zenners with their CSS problems here in the forums. You might try that.