Stories that deal with the aftermath of a catastrophic event that devastates humanity. The catastrophe can be manmade, such as a nuclear war, or a natural disaster such as an asteroid hitting the Earth or a new plague.
Source: End of the World as We Know It: Apocalyptic, Post-Apocalyptic, and Dystopian Worlds
The central themes of dystopian novels generally fall under these topics:
Government control
Environmental destruction
Technological control
Survival
Loss of individualism
Source: What is Dystopian Fiction? Learn About the 5 Characteristics of Dystopian Fiction With Examples