Theme garden will be released after 3.0 because the theme anatomy will change on 3.0
The original idea was to use 2.X base on 3.X but I have noticed that the JavaScript structure in Chevereto is hard to understand and poorly configurable, meaning that is not so easy to develop a truly unique theme. So, 2.X is not doing a good work in this issue.
Also, 2.X is showing some several pitfalls mostly because the uploader plugin (uploadify) which is pretty limited and hard to workaround even to change a background.
So, the theme garden is complete dropped until 3.X because there are several things to fix on Chevereto besides the new functions that needs to be implemented. Thanks god the PHP base on Chevereto is pretty solid and it has not shown any several pitfalls as the js.
Chevereto 2.X has shown a lot of things that needs to be worked out and I don't think that is a good time to start a theme garden.