The Community Theme Creator for BigBox is still in development and should be treated as an Alpha build, however it has enough features in place that would allow you to start building your own Themes.
(Please see Pyramids by @Rincewind)
As I have no documentation at this point, please visit my YouTube Channel as I demonstrate some of the various features.
1. Download .zip file
2. Unblock (right click on the .zip file, select properties and click on Unblock)
3. Unzip where ever you like
4. Go to the unzipped folder and go to folder ".Install these fonts first to allow the demo theme to load correctly in the editor"
5. Install the 2 fonts (these are only needed for the included Theme)
6. Click on the "COMMUNITY Theme Creator for BigBox.exe"
7. Fill out the setup properties (userid / name and location of the BigBox.exe )
FYI. If you wish to design a Theme in either 4:3 or 16:9 or any other aspect ratio then simply change your windows desktop settings to the desired resolution and open the editor.