What does the theme shift from in The Hunger Games?

The book is set in the future when North America is set into 12 districts. Each year the capitol picks one boy and one girl from each district to compete in The Hunger Games, a fight to the death on live tv, where only one survior comes out. Katniss's sister is picked to compete, but Katniss volunteers to take her place, therefore Katniss has to compete in the games.

(In my personal opinion The Hunger Games is one of the best books I've ever read. I would definatly suggest this book.)