I could be wrong, but I think the OP was asking about "CS Theory" meaning the branch of computer science that deals with the theory of computation. Turing Machi

I could be wrong, but I think the OP was asking about "CS Theory" meaning the branch of computer science that deals with the theory of computation. Turing Machines, Automata, The Halting Problem, etc. Although those things are all old. I'm not sure what's new, so I can't answer his question.