One important theme of The Stranger is life's meaninglessness, and behind this, the theme of renunciation as a path to freedom.
Meursault is a character alienated from and largely passive about everyday life. He enters into relationships with people, such as Raymond and Marie, chiefly because that other person makes it easy to do so. He enjoys sex with Marie and is passively willing to marry her, but he tells her he does not love her.
After he is tried and convicted for murder and his conviction goes to appeal, Meursault decides it makes little difference whether he dies at thirty or seventy. He reasons that we all inevitably die and that the world goes on without us. His only concern, as he tries to convince himself that his appeal will fail (which it does), is that he would feel a mad rush of pleasure at an extra few decades of life. However, he is able to quash that desire. In achieving renunciation and resignation, Meursault is then, paradoxically, able to achieve a degree of peace, knowing he is free of both expectation and the fear of death.
The absurdity of life is frequently pointed to as another important theme, and this theme emerges, as does the theme of meaninglessness, from Camus's experience of death, destruction, and ideological irrationality during World War II (as well as the absurd destructiveness of France's trying to hang onto colonial power in Algeria). This novel is one attempt to try to pick up the pieces and make sense of a world that seemed to have gone awry.