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

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.

What is the universal theme of The Stranger?

Camus' novel is expressive, primarily, of themes relating to absurdity and the difficulty of rendering life meaningful. Mersault is a character who denies conventional and formal explanations of human behavior. He does not see the same reasons for actions nor the same excuses for violence that others do. 

In fact, Mersault sees a lack of reason in his most significant act, a murder. When Mersault is captured and imprisoned, many people challenge him to accept their explanations for his behavior. Lawyers and priests provide him with stories to excuse his violence act. Mersault denies them all. 

In an expression of Camus’s humanist logic, neither theology nor fate can offer men of intelligence (men like Meursault, willing to use only bare logic to consider the question of life) an explanation for the absolutely senseless things that humans do—war, murder, and other heinous acts. The alternative, therefore, is absurdity.

We can see from early on in the novel that the world does not make sense to Mersault as it does to everyone else. Another way to say this is that Mersault sees absurdity in the world, whereas others see order, sense, and rationality. 

For Mersault, reason does not guides men's behavior and certainly does not guide his own when he commits a murder. As a result of this lack of reason, Mersault can draw very little meaning from the act. Where there is no reason and no rationality, there is no meaning. 

Articulated as themes, these ideas of absurdity and the difficulty of meaning can also be related to personal inability to construct a meaningful worldview or philosophy. Mersault is a man beset with absurdity. 

What are the major themes, techniques and purpose of Camus’ The Stranger?

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.  

Is the main theme of The Stranger by Albert Camus existentialism?

I would say it does have an existentialist theme but don't leave out Absurdism (check out link below).  Camus did not consider himself an extentialist; he considered himself an Absurdist. There are many similarities, so it is not some outrageous move on his part. But to be sure, there is one key difference.

Existentialism means "existence precedes essence." So, you are born a blank slate (tabula rasa) and you are responsible - and free since the responsibility is yours - to create, construct meaning in life.  There is no inherent or essential meaning waiting for you to stumble upon it except via faith. There are no great truths that have always and will always exist. The meaning in your life is created by you. The pursuit of meaning may or may not have meaning.

For Absurdism, all these attempts at meaning are possible, but essential meaning (which we don't create - the eternal kind like Absolute Truth) is simply beyond human comprehension - if it exists at all.  The Absurdist embraces the absurdity that meaning is only possible in general or in the pursuit of it.

Freedom is a main component of Existentialism and Absurdity. Meursault is completely free (mentally) because he embraces the Absurdity of existence. He finds meaning only in what is immediately present to him. Eternal truths offer nothing to him. Human values, he believes, only serve to keep people in line and thinking the same way, blinded by an idea of universality. Here's the key point of existentialism at least with how Meursault is concerned. Meursault is never persuaded or conditioned or convinced to act in a certain way by any other character or any other belief system (think of the sections with the priest).  This is existentialism: find your own way, even at the risk of totally alienating yourself from society (which he does physically - prison - and psychologically all along).  Meursault's essence is all his own doing.  As stark as it is, he didn't even let imprisonment or threat of execution dissuade him from acting of his own free will.

What themes does Albert Camus convey in The Stranger and how does he do so?

Albert Camus’ novel The Stranger explores everything from identity to family to gender to economics. Let’s look at some of these themes as well as some of his narrative techniques to get you started on this assignment.

In terms of identity, you should talk about Meursault’s detachment from everyone, including himself. He cares about nothing, not his family, not his job, not even himself. He is very much a cipher, and you should explore how this affects his identity and self-awareness.

As for family, we have a contrast in this story between Meursault’s uncaring attitude toward his mother’s death and the grief of people who are not even relatives.

Gender and nationality play a role in the tale, too. You might explore the relationships between Raymond and his mistress and between Meursault and Marie and discuss how women are treated. Since Meursault lives in Algiers and is a Frenchman, the themes of colonization, discrimination, and prejudice play a role in the story. You should discuss these. You can also focus on Meursault’s social class and how that affects his perspective and decisions, including his indifference.

As for Camus’ techniques, you should make note of the detached tone he uses to tell this tale, a tone that mirrors his protagonist’s mindset. Look, too, at his use of verb tenses, which are largely in the perfect and past perfect rather than in the simple past. This allows for still greater distance, as does Camus’ choice of descriptive words with regard to emotions, actions, and even setting. He is trying to reflect the meaninglessness of the world of the story, at least in Meursault’s eyes.