Re: CSS files. . .
I recommend the web developer toolbar add on for Firefox. It will let you change font sizes and colors on the fly temporarily, then, when you’re done, you can make those changes permanent in your style.css.
Regarding the display of items on your forum, either to add to them or remove from them, I usually search through the files in my template folder for a specific piece of text that’s being displayed. Normally, that shows you right where the template is generating that, and you can either remove it or add to it there. That’s a start.