What best describes a central theme?

I always found nature's indifference to man as being a central theme in the story. 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.