Category Dressing: Defines question

Just installed Category Dressing. Great piece of work, IMHO!

I was able to set the Categories menu to exactly the same appearance as my original HTML/CSS design in about 5 minutes!

But - although I have been staring at the categories_dressing_defines.php file for a while now, I just can't seem to get my head around how it works.

I can't see how the definitions get placed on the page. I see the styling in the CSS file, but I can't figure out where the code is referred to so it appears on the categories menu.

I just want to take out most of the headings and style the few that I need - the top "Categories" head and maybe one after the entire list of categories.

Sorry for asking about what must be a pretty obvious thing - but I can't seem to figure it out.