What are main themes in Jeanne DuPrau's The City of Ember?

The central theme in Jeanne DuPrau's The City of Ember concerns the need to act against oppression. Ember was established as a city of refuge to preserve the human species in the event of an atomic war or other apocalyptic event. But it was only designed by scientists to last about two hundred years. After that, the citizens would need to leave the city, and the Builders needed to be sure to leave instructions for the citizens to find their way out. However, as time passed, the instructions were lost, and the government became corrupt, tyrannical, and oppressive. The people particularly suffer under the current mayor, who is hoarding provisions for himself rather than finding a way to help the citizens. Throughout the story, DuPrau explores multiple responses to oppression and promotes Lina's and Doon's response as the most needed and most successful.

DuPrau shows that most citizens of Ember are willing to do nothing so long as they continue to have what they need to survive. She also creates the Believers to show that others approach oppression by establishing optimistic yet inactive belief systems such as the belief that the Builders will return to the city to rescue the people, as we see when Captain Fleery explains to Lina:

I know it in [my heart]. ... And I have seen it in a dream. So have all of us, all the Believers. (Ch. 7)

In contrast to those citizens who take an inactive response towards the situation, Lina and Doon rebel against the government by feeding their curiosity. Neither character is content to simply accept the government's word that everything will be alright. Instead, they explore their surroundings to look for a solution on their own. Doon is especially convinced the solution can be found in the Pipeworks and uses every spare minute he can to explore on his own. Because Doon and Lina are willing to actively rebel against the oppressive government, they are the ones who successfully rescue themselves and the citizens, underscoring DuPrau's theme concerning acting against oppression.