Is there an active theme thread going for CM7?
Yes. On the third page of this forum you will find this:
CM7 Theme changer themes.
It is a link to what is happening over at XDA. As CM is changing and not released as a stable platform, the themers that are doing stuff are primarily using the TMobile theme engine built into CM. These can be found over at XDA (in the theming section) or in the app store under CM Themes or TMobile themes, etc. It's easier to find the goodies on XDA. And when you find these, our device is HDPI. The themes are installed as an application, then chosen in the Theme Chooser application on the phone.