Clearly, any work of literature has a number of themes, and this novel is no exception. However, one clear theme to me is the way the novel explores morality and the various moral codes that the characters in this novel ascribe to. Let us consider Jake for a moment, who describes "morality" as "things that made you disgusted afterward." He quickly corrects himself, saying that this must be the definition of "immorality." However, throughout the novel it is clear that Jake is more interested in his own problems and issues, and then, only secondly, those of Brett.
Now, if we compare Jake's moral code to that of Cohn, there is a stark difference, as Cohn shows himself unable to understand the morality of Brett and also unable to follow it. Note how Brett and Romero are got together, deliberately going against the morality of the group. This of course leads to a betrayal and the loss of respect between Jake and Montoya and Cohn. Jake shows how throughout the novel he continually becomes more and more self-centred and selfish.
If we consider Brett for a moment, we see that she is presented as wandering in a kind of moral vacuum. She shows self-disgust and also disgust with Jake, even though this is shown to be unfair. She stands against conventional morality by engaging in numerous brief affairs which shows her to be a self-destructive personality that is often presented as being passive in the novel. These string of affairs are of course an escape mechanism to flee from her relationship with Jake, whom she really loves but is unable to have sex with her. The one time in which she does morally assert herself is in disposing of Romero.
Thus morality is an important theme as characters reject conventional forms of morality and build their own system of morals, justifying them as they go, for their own reasons. Such morals have a lot to say about the characters in this book and the kind of desperate, detached and selfish lives they lead.