It's a long time since I studied some graph theory in a combinatorics course, it wasn't useful for games, more about properties of certain graphs (how many trees with N vertices are there? Is this graph planar? Graph colouring stuff (vertices and edge colouring only), Hamiltonian graphs (graphs where you can visit every vertex without traversing the same edge more than once), etc.
It wasn't too hard though so if you want a break from complex analysis I'd recommend it ;) EDIT: And topology. That was really hard as well ;)
EDIT: CS graph theory is probably useful though, I'm talking about combinatoric graph theory.