what is the theme of the maze runner?

I might consider the apocalypse / environmental Collapse. Though none of the Gladers can remember what happened to them before entering the Glade, clues suggest that a cataclysmic event took place in the recent path. As a result these children have been rounded up for participation in a cruel social experiment to test their will and perseverance, as well as their intelligence and ability to think and act under extreme pressure.

While it is not clearly expressed what exactly took place in the past, the Gladers suspect that the world as we know it no longer exists. As more of them go through the Changing and remember their past lives, this suspicion is confirmed. Alby is so repulsed by his memory of the real world that he has no desire to leave the Glade. When the Gladers do eventually emerge from the Maze, they see the effects of the Flare and the ravaged landscape. This setting, common in science fiction and fantasy, presents a world without order in which conventional ways of life have been all but eradicated. Humanity is reduced a struggle to survive. It functions as a sort of "re-setting" of society and civilization.