Would you say the main theme in The Stranger by Albert Camus is 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.
Existentialism is definitely one of the themes of the novel. You can tie that into how Merusault is a nihilist and fatalist. In addition to Merusault, Salamano is also a good example to use. After his dog dies, he wanders around with no true purpose and questions his existence.