The usage that I thought was standard was to use "decision theory" and "game theory" synonymously and then use "combinatorial game theory" to refer in particular to the games which are deterministic and have perfect knowledge e.g. Chess, Tic-tac-toe and Nim. It's combinatorial game theory for which Conway used the theory of the surreals, and I haven't heard of any use of them in game theory outside of this.

EDIT: On second thoughts, "decision theory" and "game theory" aren't synonyms; game theory is the subset of decision theory involving interactions with other agents.