It can be done with css only, but the result would be limited to what you can achieve with css. For instance, the header has a background image, that you could replace with just a color, but that wouldn't look as good as it could do.
Therefore, a combination of images and css is always advisable. Please download the psd source files as well and edit those to your liking. And use firebug to check out the css and make changes to have the colors suit your preference.