Manipulation is one theme, but this thematic concept doesn't become clear until later in the story. The Maze, the Glade, the Grievers, etc. could all be natural parts of the world . . . or supernatural. But at the end of the story, a group of survivors finds out the entire thing is one big experiment being run by the Creators. That's manipulative. Put a bunch of kids in a box with walls that move around and see how they react.
I'd support a second theme of freedom and danger vs. confinement and safety. There's a constant tension between the boys about what to do in their present situation. Some believe that the Glade is fine. It has what they need to survive -- food, water, and shelter. Any dangers present are known dangers. On the other hand it sort of feels like being held captive. The alternative is to seek freedom by running the maze, taking on the Grievers, escaping, and finding the completely unknown.
For a symbol, I'm going with the sun. The represents consistency and normalcy to the boys. It's always there. The maze may change. The boys in the glade may change. The rules of the creators may change, but the sun doesn't change. It's consistent. It's always there. It's also symbolic of hope, but that symbolism doesn't become apparent until the sun is taken away.
"In other words, the sun that had shone down on these people for two years, providing heat and life to everything, was not the sun at all. Somehow, it had been fake. Everything about this place was fake."
"Everything about this place was fake" is a dire and depressing line to read. It feels so hopeless. Everything that the boys have come to know and assume was real, isn't. It feels as if their entire world has been flipped. And not in a good way.