I have been trying to get this done for years and finally was able to make it happen. I have successfully managed to custom edit existing menu themes. The key is you can't change the theme name, so pick one of the themes you don't really use and go to town. To modify the menu you need to work with the groups that are inside the theme. I wanted to add 4 more lines so I could have 10 text lines on a Blu Ray menu. You can see in the attached image that I was able to do it. The Theme files are located inside: LIbrary/Application Support/Roxio/Toast Menu Styles.
This is where I selected Layout 6 under Main Menu and right clicked and chose DUPLICATE GROUP, then renamed it to Layout 8 and did the same for Layout 10. Then I had to duplicate the Button Groups inside the layout to add buttons for 7,8,9 and 10.
This is how the 10 movie looked after I was all done. You can change the background image the font etc.
Here's the final result .. this is a screen capture from the actual burned Blu-Ray disc.
I have done this editing to other themes as well so I can have 1, 2 and 4 items to the right so I can have a custom background image.
Here's a snippet showing 4 images offset to the right.
If you have any questions you can email me at: firstname.lastname@example.org