What are the major themes of The Stranger by Albert Camus?

One of the themes of The Stranger is human alienation from oneself, each other, and from society as a whole. Meursault, the protagonist, is a symbol of this alienation because he is a Frenchman living in Algeria, a Muslim country in which he does not really fit in. He is also so alienated from himself that he does not mourn his mother's death at the beginning of the novel. The novel begins, "Maman died today. Or yesterday maybe, I don't know" (page 3). He is so alienated and distanced from his emotions that he is unable to mourn his mother's death. 

Another related theme is the meaninglessness of existence. Camus was an existentialist writer, who believed that humans must make their own meaning in a world that is essentially without meaning. The Stranger has many absurd elements to it, such as the way in which Meursault's response to the magistrate waving the crucifix about his head is to think, "I was hot and there were big flies in his office that kept landing on my face" (page 68). The magistrate tries to convince Meursault that God will forgive him for committing a murder, but Meursault finds the magistrate's reasoning absurd and meaningless because Meursault does not believe in God. Even the murder that Meursault commits is meaningless, and he does not provide any reason for killing his victim aside from his feeling pestered by the sun. His actions are without reason, as he feels as though he lives in an absurd world in which nothing is governed by reason.