I suppose (ideally) we want a site like this where users can preview the themes and download them, and perhaps an additional 'Edit' button. This would lead to a separate page to allow the editing/creation of a new theme (based on one chosen from the front page). Once edited they could then download their handiwork a-la studiostyl.es. Edited: But having just read recent posts, I realise this is a little ambitious at this stage
If only there were an XML expert around who could create an XSLT - such that we could use/borrow/picky-back studiostyl.es and just transform their XML to a ST-theme file