I would suggest you start with the default theme and take out all of the CSS. Of course you can always add back the parts you really need. That gives you a blank slate and makes it easier to add a background, for example. It may sound extreme but I personally think it's the easiest way to learn how to theme WordPress with CSS.
Reverse-engineering a theme is difficult because there are more than a dozen ways to add a background, so there is a lot of detective work involved to get into the mind of the theme designer.
Either way, get "Firebug" for Firefox, also Google Chrome has something similar to Firebug, this will help you see how a theme has incorporated a background, for example, these tools can help you locate the exact CSS you need to change, assuming you're not able to understand the CSS on your own.