The theme isn't a concrete rule in how it's implemented. If you're interpreting it as two button inputs from the keyboard, then I would probably say that adding other keys for non-gameplay purposes would be more than fine.
I went with two physical buttons represented in my game as stamps but I was planning to also use keyboard binds for fullscreen, close, just not as core gameplay controls beyond inputs to select one of the two physical buttons. I do have the player click on customs documents to select them before using one of the buttons as well.
