Firebug makes it very, very, very easy to find out exactly what elements you are working with. I do a lot of custom CSS and without Firebug I would have to do a lot more guesswork and spend a lot more time. I highly recommend that anyone interested in altering the theme download Firebug (it’s free) and spend a few minutes learning to use it. The learning curve is very gently, believe me.
If you use Google Chrome, it has a similar feature already built it. Go to Tools >> Developer Tools, and something similar to Firebug will appear.