One of the dominant themes of The Stranger (also translated sometimes as The Ousider) is the absurdity of human existence. Mersault's life simply collapses piece by piece, with no rhyme or reason, and he seems powerless to do anything about it. Another is the colonial relationship between France and Algeria. The book is set in Algeria, a French colony. Mersault interacts almost entirely with French-Algerians, and murders an Arab on the beach. Another is free will. Mersault seems not to care about the universe, and the universe doesn't care about him. He does things to others, and things happen to him without rhyme or reason.
These themes are typical not just of Camus's work, but of existentialism in general. Existentialism was characterized by the belief in absolute free will, in the absurdity of believing in any plan for the universe, and the rejection of dogma, including religious dogma. This sensibility is brought to the surface in Mersault's confrontation with the chaplain who comes to pray with him before his execution for murder:
I started yelling at the top of my lungs, and I insulted him and told him not to waste his prayers on me...He seemed so certain about everything, didn't he? And none of his certainties were worth one hair of a woman's head. I had lived my life one way and I could just as well have lived it another. I had done this and I hadn't done that...And so?...Nothing, nothing mattered, and I knew why. So did he.
Camus's style, in which Mersault is the narrator, contributes to the sense of the absurd. Mersault does not comment on his behavior, nor does he judge the behavior of others.