If you don’t know about W3schools.com then do yourself a favor and go there. Especially if you are trying to do something in CSS and you aren’t sure how. Not where, mind you. In Drupal, that is your own thing to find. But as to how, W3schools’ web site has some of the best free references around. And the Try It features are the best. Alter their examples to see if you are getting the results you want, then put the changes into Firebug to test on your site, then alter the actual CSS files and test your live changes.