My SOP is essentially a research proposal. It is a document about the future but I do touch on my undergrad thesis, a research gig, my current job, shit like that. I'm still sorting out how to approach theory in my SOP. 2 of my 3 LORs think theory should be up front and prominent but not necessarily more than a nod. I'm still figuring out how to nod clearly and subtly. I am also applying to theory heavy schools with a theory heavy project. I've been told at UChicago you can easily end up reading back to St. Augustine in order to make sense of anthropology's place in western thought. I think that's pretty great. I'd read Augustine over Boas any day of the week.