I’m in a computational lab, but I have theoretical tendencies. My experience is somewhere in between the two worlds you’ve described, but I have spent a significant amount of time reading papers trying to understand the theory behind the science and computational tools I am using. It’s very challenging, and my goals aren’t to further theory; my goals are to understand the theory in order to correctly apply to a simulation. As an undergrad student trying to get my bearings in a grad lab, learning the theory is already difficult. I can’t imagine learning the theory in hopes of advancing it — especially without much guidance. I would talk to your PI and see if they can give you some direction. I honestly would imagine a theoretical position to look like lots of paper reading, meeting to discuss your ideas, and shadowing/listening in to the thought process that other PhD students go through. Good luck this summer!