Have time on your hands and want add a unique yet mediocre theme ("accent color") to your HTC device? Well, as you can see above I overwrote my Blue theme for a gray one. (I actually had no idea it would over-write the Blue theme, otherwise I would have chosen Brown).
Of course, gray is just one of many possible themes one could do, so this was more conceptual than anything. The story of it goes back to XDA, involves TouchXplorer (a file explorer for unlocked HTC devices) and a provisioning trick detailed by xbodmod. We won't go into all the details because to be honest, it's way more effort than it's worth, unless you have some gray fetish.
ChevronWP7 was saying they would have some sort of solution to adding or changing themes, something that was presumably easier, but so far that hasn't happened and we're not holding our breath. Finally, while an all-black theme sounds bad-ass, it's actually not so great. Best bet here: just wait for your carrier and/or Microsoft to give some more "themes".
Update: Neowin has a nice step by step tutorial on the matter.