Almost all of it is done in CSS. You can change the gradients dynamically if you'd like to change the color scheme and want to dive into the CSS itself. It is not very complex, actually.

There are some cases though where an image is needed. This is only for IE however, which doesn't support CSS gradients (there are proprietary workarounds, but I did not use them in the theme). The navigation bars have such "fallback" images, so you would have to want to change them as well if you wanted to make sure that any color scheme changes were reflected in IE appropriately.

Things like shadows though are all done with CSS. The cyan navigation glows are not.