The lighting is very different from the scene in Port Royal when Jack get's introduced. There, it's the middle of the day, it seems warm and tropical. The lighting is very bright, and it feels safe. In this scene however, the lighting is almost non-existent. Only a few torches(which for some reason never go out), a full moon shining outside in the sky and occasional glints of treasure when light from either the torches or moon hits it and is reflected off it provide all of the vision in the cave. Altogether it makes the atmosphere quite eerie and the presence of a full moon may symbolise bad luck. But the main point of the lighting is to make the pirates seem evil and scary, since this is their main home and base, as well as make the scene more tense. This adds to the audiences minds about how clever Jack is in tense situations because darkness is usually associated with fear and the fear makes the scene tense.