hi moondancer.. as "A" noted you'll edit the css to do that. You'll also need to edit some images, upload them to something like Photobucket and use the urls from there (replace them in the css) Mostly the main_block.png image and there are MANY places in the css that you would have to replace that image url. Im afraid its very confusing to explain and there is not a tutorial for doing that. If you know css and are good with graphics and can weed thru it to figure out what things cover what it CAN be done.
Ive attached the main_block.png file so you can take a look at it. The images in that one file are used throughout the forum. You would need to change the colors in that image but would also need to change some color codes throughout the css for areas that use that image.