The only way to currently make your Twitter unique through customization is by adding Twitter Backgrounds. You do not have much choice beyond the 12 or so twitter features. You will need to go to the many external sites that allow you to download them especially for Twitter for free, then customize the CSS. One good site that gives you the codes with the backgrounds so you can do it all yourself is PlayWithMyTwitter.com. There are many others, though that is probably the fastest-growing one at the moment.