Does anyone know a way to create custom themes on iDVD 08? Let me know.

Found after googeling a bit (not mac user), maybe you can have some use of the info:

drag and drop your JPEG, GIF or TIFF file into background box in the Settings Pane from the customise draw along with dropping your file into the Project file folder within the Project map.

If the Theme templates are still visible over your custom theme then re-drag your custom file back in the Background box within the Settings Pane – this will automatically knock out the original theme!