I think you could approach answering that question from many different angles, for me complexity is a structural feature of systems, it is a systemic feature and thus to talk about complexity you really need to talk first about systems, create some model for what you mean by a system and then start adding complexity to that. So maybe the answer to the question is that whereas systems theory deals with all kind of systems complexity theory helps us to understand systems that are complex, in that sense one could think of it as a subset of systems thinking.