What is the theme in The Dark Is Rising by Susan Cooper?

The battle between the Light and the Dark is an eternal battle. "But always the Dark was there, swelling and waning, gaining a new Lord of the Dark whenever a man deliberately chose to be changed into something more dread and powerful than his fellows." It is the classic battle between good and evil. The agents of the Dark represent evil, and the Old Ones, and tools of the Light represent good.