One great feature of CM7 is the built-in theme chooser. It allows you to download CM7 themes from the market and change them on the fly! With all of the existing CM7 themes out there, we already have dozens of themes right off the bat...
NOTE: Your notification widget won't show up after changing a theme...try Notification Restart from the market, or reboot after choosing a theme for the full effect.
Recommended: Download Theme Chooser Wallpapers from the market to access the built-in wallpapers in many themes
Here's a good list of themes...and most themes have a donate version, so make sure to donate for the ones you like
Here is also a link to an xda thread that has a great list of themes!
JAMT - Flat Black
JAMT - Gradient White
Dreads Black & Blue
Smooth Blue by trenton
NTSense and NTSense Blue
Sony, Samsung, Motorola Themes
EasyDay <-----Choose HDPI
Dread's Black and Blue
iKaNDee, GiNGeR3Ds, PURPL3MiX
VA Dark Orange Port <-----Choose HDPI
Rover Mix, Rover Blue, Rover Cyan
Cyan Skulls and Dragons
Mixer Theme <-----Choose HDPI
Meizu <-----Extract, and install HDPI