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.