been looking and can't find any CM7 themes for the D2G
[CM7 Theme Engine][HDPI/MDPI] Mixer Theme..Updated 7/26/11..Now with Purple/Red. - xda-developers
Just install that apk, open Theme Chooser and select the theme and it chages instantly. Some themes require you to reboot to take full effect.
Search the market. Try honeybread and minimal bread. Both my favorites. Then you can branch out from there from the market links (view related apps)