What is the theme in American Psycho by Bret Easton Ellis?

Some of the themes are:

Death/Oblivion: Although the work is satirical, the scenes of death are often brutal. This, matched with the oblivious nature of most of the characters, creates a duality with oblivion, both in the sense of loss of life (which is left open at the end as to its reality) and of people's callase and lack of awareness.

Shallowness of Society: The book is intended as a satire of the 80s, a time of plastic clothing, bright colors and big hair. Drug use is as regular as eating and no one seems to know or care who anyone else is.