YouTube lets you “turn the lights off” and countless sites allow you to change your background, if not more.
Thanks for your suggestion for custom meta theming. But my stronger interest is for user selectable theme for my (future) site.
Presumably DC will ship with the Darkness theme available for the admin to select. Why not allow the admin to make darkness (or other approved schemes) available for their users to select as a preference? Is it because @codinghorror abhors it? 
I doubt even 1% of my users know how to override the colors, and only a few more would take the trouble to do so if they knew how. But a much larger percentage (25?) would select an optional theme if provided as a preference.
I hope that my site will be my one of my users’ online homes. Being able to personalize it a bit makes it much more “homey” and encourages them to take up residence (and be on ongoing contributor).