Start by creating a duplicate of an existing theme for a template. Then, rename it and open up Photoshop and start looking through the layer folders to see what you can customize. The layers are well named, which makes them easy to find and manipulate. Some important notes:
- Background layer: This is the layer you want to start with. This will be the background for every menu choice throughout your DVD menus.
- Main Menu folder: Within the Main Menu folder, there are many subfolders. Each one relates to the menu Toast uses depending on how many clips you will be burning on your DVD ("Layout 2" for two clips, "Layout 4" for three or four clips, and so on). There are also layouts for albums, artists, and songs, which are for the music DVDs that Toast can burn (I haven't tested these). One addition from Toast 6 is the Preview layer, which Toast uses to display a preview of the theme -- so you'll want to make a copy of your completed theme and replace this layer, too.
Finally, one shortcoming that I have yet to solve: Although Toast automatically recognizes that there is an additional theme available, and initially shows the correct filename, once you click on the theme, it reverts to the name of the theme you used as your model (in my case, Adventure). While not a big deal, I'd rather not have the same name over and over in the list. If someone figures this out, please post!

