What is the theme of The City of Ember?

The City of Ember, published in 2003, is the first in a series of novels for young adults by author Jeanne DuPrau. There are a number of themes within the story, but here, I will take a look at the importance of the theme of greed.
Many of the characters are guilty of greed in The City of Ember. Mayor Cole, who hoards things away (light bulbs, to be specific), is especially guilty. But, when the Mayor is reported to the guards, it turns out that they are also benefitting from the mayor’s hoard and are as corrupt and as greedy as he is.

He could think of only one other possibility. The guards . . . at least some of them . . . already knew what the mayor was doing. They knew about it and wanted it to stay a secret. And why? It was clear: the guards, too, had been getting things from the storerooms.

Looper also is guilty of greed, as he steals things from the storerooms—things that could help Ember as a whole—and he sells them for huge profits in his own store.

But Looper’s stealing. And Lizzie, he isn’t just stealing things for you. He has a store! He steals things and sells them for huge prices!

Even the sympathetic character of Doon is guilty of the sin of greed, as he desires recognition as a hero from his father.

He wanted to shout to his father, we found the way out! We’re saved! But, he held himself in, for the sake of his own glory.

One of the book's themes is that people must take control of their own destinies if they are to change anything. Life in Ember is rigidly controlled and so tightly structured that it encourages a mindset of passivity. It is only when characters such as Lina and Doon actively seek to find out how to save their community that change finally comes about. In such a dystopian society, most people just put their heads down and try to live as best they can. But Lina and Doon are not satisfied with this. They are fiercely individualistic and know that life can be better for everyone—but only if they actively challenge the existing system and discover the truth about the City of Ember.