Well that didn't take long. The SageTV HD Theater (read review here) has it's own, unique user interface for standalone mode (when not being used as a SageTV extender), but it can be customized just as the SageTV UI can be customized. Until today no one had released one, but as of now the first custom UI is here.
Gerry, the user/developer for many SageTV add-ons including the SagePro Theme has created a SagePro Standalone UI to match.
Instructions for installing are very easy and it looks quite nice. Watch for many more custom UI's for the SageTV HD Theater Standalone in the near future! Thanks to Gerry for his work on this and for being the first one to do this.