If you explore the program data for DVD menus you will find each has a folder with several files:
The. bmp files are no problem as they are the images you see for the layout menu and the preview screen. The .tpl file is in plain text, but whether you can make sense of it is another question!
The _msi_keyfile_... is beyond my comprehension.
And then, even if you sort all this out for one menu, you would have to integrate your new menu template info into the .ini files and the masterlist.csv further up in the Windows file system. And the above examples are for a plain default design.
You have quite a lot of control of the items in a DVD menu in 'Edit' more (button in the top left of the DVD burn screen) and also some editing in individual buttons using the Edit option at the right:
You can of course save any modifications you make so they can be used again.
NOTE: the above images are from the '...DVD>CompLayout' folder. Have a look at what is in 'DVD>Layout' for all the bells and whistles and buttons!
I agree with Carl. Designing and integrating a brand new template from the ground up is not an MEP option. The only practical approach is to use In arranger: