Where is a metaphor in the city of ember?

City of Ember have to grow well in an amazing world of glittering lights and quiet happiness. But when the cities large power generator begins to fail. The street lamps start to fade-along with the hopes and dreams of the towns people. Now it's up to two courageous teenagers (Lina and Doon) to follow a trail of clues left by the ancient builders and find a way out of ember before their world becomes into darkness forever!