Ubuntu was famous for being brown, even though it was probably half-orange for most of its storied existence. Mark Shuttleworth and Co. mostly blew that notion out of the water in Lucid Lynx (10.04 LTS), which is purplish and dark. I'm pretty simple about these things, so I looked at what came with the Lucid install and ditched the default Ambiance theme in favor of Radiance. I also dumped the purple wallpaper by clicking on the Background tab and selecting the Cosmos slide-show background, which not only presents a nice outer-space view but periodically changes the image (hence the "slide-show" portion of the name).