Can anyone tell me what each thing in style.css affects? I don't want to change anything but forum colors so please help me.
What exactly are you trying to change? If you can give me a little bit more info, I should be able to find things for you.
The easiest way to figure out which style attribute does what is to view source and compare it to the style.css file for the theme you're using.
Indeed when the forum is open in your browser use the "view source" option of your browser to see the css style classes in action.
For the "classic" theme you could alternatively make use of the good old YaBBSE template tools, link is here:
http://www.yabbse.org/community/index.php?thread=12659.
Css is wondefull thing, you can do a lot.
just look in the tutorials here, you could be moving up with them.