what is the main theme of the book?

I think that survival and naturalism (the great force of nature over human beings) is at the heart of this story. Naturalism not only maintains that the environment is deterministic but indifferent. The environment does nothing to help its inhabitants; in fact, it is coldly indifferent to their existence and struggle. In "To Build a Fire," the Yukon would be bitterly cold without the man, as well, and it does not cease when the man struggles to stay alive. This indifference makes survival itself a critical goal for naturalist characters. As the story goes on, the man changes his goal from reaching the camp, to warding off frostbite, to merely staying alive. Naturalism thus elicits profound conflicts, man versus nature being one of them.