http://docs.sourcebans.net/guide/development.overview
http://docs.sourcebans.net/api
As you said, it's early, haven't had much time documenting it further, but the basics are there.
As for the theme, a couple of people have offered to do one, but only one person has actually started and is making good progress. The current theme is kind of complex due to the heavy use of Yii widgets and JavaScript, so you'll probably have to get into the Yii framework a bit if you want to start on your own theme.
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