Lynne has been extremely helpful in this thread with CSS debugging, but please realize that each forum is different based on what specific styles you are using, so small edits to Lynne's CSS that work for one forum may not work as well for another.
I do recommend that you follow her advice and download either
Firebug or the
Web Developer plugin for Firefox. Using the Web Developer plugin, you can edit CSS "on-the-fly" to see what certain CSS changes look like instantly on your website without changing any code in the admincp.
Once you find the CSS combination you like, just copy the code from Web Developer and apply it to your CSS files.
Besides being an excellent tool (that does much, much more then just edit CSS) it helps you learn by doing. You see instantly what your file edits do and how fun it can be to style your website on your own.
Again, thank you for this great tutorial Lynne.