There's Sony's DVD Architect theme specifications (PDF). It's old, and the theme format changed slightly when Sony added the ability to save a menu to a theme, but for the most part I believe it's still accurate. And it's pretty much the only documentation -- Sony- or user-created -- about DVDA themes I've ever seen.
I'm surprised that theme editing is so (apparently) unpopular in DVDA. Are most DVDA users content with the default themes? Or are there other ways to create lots of DVDs with the same look and feel?
Think I fixed the link. Something about quotes in a URL that the forum doesn't like.
I'm surprised that theme editing is so (apparently) unpopular in DVDA.
Well, I'm sure the DVDA users themselves would love to make themes more often, but Sony hasn't made it easy for them. The procedures for starting from scratch or revising other themes is pretty unwieldy, and the "save menu to theme" function isn't particularly handy. So most people either make due with the themes we have or build their menus in Photoshop. The design I want for a project usually depends on that project, so I don't use themes much myself.
As for other ways to create lots of DVDs with the same look, another way would be to set up a menu the way you want and save it to a DAR file. When you want to use that design in a project, open up that file, add your content, and save it to a new file. Kind of like a template rather than a theme.
You have to be careful, though, deleting old content from a DAR file in order to make a new project -- DVDA has a habit sometimes of getting annoyed when it can't find media that used to be there.
If it helps, I think the biggest limitation to DVDA's themes is it can be harder to get your head around the DVDA principles of thumbnail & frame masking, & DVDA themes are a bit limited in how they save button placement, which can be a problem particularly for scene menus. IMHO it's something that was there originally, just never took off in popularity, and so just sort of got carried over from version to version almost forgotten about. No matter what you do for things like scene menus, you'll still most likely have to drag buttons around.
You can also save templates or graphics files to use as templates in your graphics software. Some of the vector graphics packages are pretty handy for this sort of thing too. Either vector or image editing software should easily handle placing images inside shapes or objects, &/or you can generate backgrounds with cutouts to use in Vegas or other editors. In Vegas you can also save projects to use as a template, but you have the necessary option to remove media from the project. You can save libraries of shapes to use as frames inside of DVDA, &/or or just create a file from an image to be used as a guide, to use as a temporary menu background - replacing it in DVDA once you drag all your buttons where they should be. That comes in handy as it's often a pita in DVDA otherwise.