Numerous ideas are raised and treated in Synge's The Playboy of the Western World. In complex literature, there often isn't one theme that dominates over others. That's probably the case here.
The enotes Study Guide points out two themes: Social Conventions and Rebellion. I'll let you read about those in the Study Guide. In short, the villagers first make Christy a hero for his act of rebellion against social convention, then condemn him when faced with the reality of a son attacking a father. The villagers also, for the most part, are trapped in social conventions themselves, though they at first praise Christy for rebelling against them. Only Pegeen is truly a nonconformist.
In addition, the play deals with the creation of myth as well as violence in Ireland, or more specifically, the love of or at least the fascination with violence.
The Irish people, some would say, are not presented in a positive light by Synge. They make out Christy to be a hero for having comitted a hideous act (how myths are started? is this what Irish heroes are actually like?). They are fascinated with the "heroism" and "bravery" of a son who kills his father. When the myth crashes and Christy is revealed to be a baffoon, the villagers, too, react with violence, almost hanging Christy. The Irish, it would seem, are prone to violence.
If you must choose, study these themes and choose for yourself. One doesn't really stand out above the others, although the idea of myth making is probably a little less present. You could make a case for any of the other three.