How is your game related to the Theme and Special Object?
It's pretty clear where the cardboard box fits in, as the main point of the game is sorting out cardboard boxes. The theme "don't trust everything you see" is present because the cardboard boxes are opaque so you have to be careful about which ones you let through. In addition, because of the unpredictability of the physics based factory you might accidentally let through a box that wasn't allowed so you have to have a healthy level of suspicion of every box at the end of your line.