The theory posited in The Stranger, boils down essentially to the idea that nothing matters.
I don't matter. You don't matter. The keyboard that I'm tying this on doesn't matter. It's all just kind of there and kind of in free fall with the absense of a governing body to watch over and manipulate it all.
Mersault embodies this line of thinking, wading casually through a lifetime of nothingness, passively assisting in the petty foibles of quasi aquantences and always telling the truth without fail. He renounces religion and kills a man simply because the sun is shining down so brightly upon him.
Still though, he enjoys life and doesn't want to die once the death penalty is placed upon him. It's the little things- amusing, through still ultimately irrelevent- that give him joy. Swimming in the ocean, a nice hard cigarette and casual sex with an old aquantance from work do bring him joy. If we take The Stranger's thematic underpinings of absurdism and nihlism to heart and examine the little things that cause nominal, irrelevent joy, I think Art would pop up somewhere in that conversation.
If life doesn't matter, than arguably art, which theoretically reflects life, wouldn't matter much either. The Stranger is a devisive book and your enjoyment of it is determined largely by your philosophy on life. A religious person or someone inclined to believe that life is, you know, meaningful, probably wouldn't that be turned on by its concepts, whist a nihilistic, or even realistically based person might find it amazing. What's great about art is that no matter where exactly you fall on that sldiing scale of philosophical beliefs, you can enjoy and appreciate art. If everything in the world matters, than experiencing art can be a deeply personal exploration of the mind and of the soul. If you think it doesn't, then reading a book can be an amusing enough way to kill an afternoon or an evening. Either way, it works.