I think you have two choices here. It's more about concept than about layout.
If you think you're simply adding, as a new layout option, theater mode as a tab, then using default theater view file would suffice.
But I feel what Inminente requested was more than that. It's highly easy customizability.
I'm thinking about something like this. You right click on the default theater view panel (as a tab), then you have a list of elements such as gallery(artist, composer, etc.), artist info, lyrics, spectrum visualizer, sound graph, progressive bar, etc. Upon choosing one, you have a drag-and-drop panel or element, not only the place but also the size and color of which can be easily adjusted by simple mouse movement and the color selector. As you know, the shape of Playing Track tab is so variable and customizable that there can't be a single layout fitting nicely.
As for the background image, I think the current no-image background tone of Playing Track tab would work nicely since the overall tone of theater mode as a tab would be highly skin dependent.