One of the major themes of the story is absurdit. Camus builds this idea up with the idea that as a person grows wearily accustomed to a mind-numbing routine, that one day they may actually begin to question why. This is the start of all things he says as the approach to life is then one of a person who questions all things with a touch of the absurd.
Another theme that is present is that of colonialism or perhaps just a more simple racism. Meursault struggles with the idea of being a white man among those of a different race and it presents all kinds of difficulties for him not just in terms of actions but as well in the way he approaches life.
Another theme is the idea that Meursault has of free will, in which he is concerned only with making decisions regarding his own comfort, etc. He is not willing to subscribe to any particular beliefs or moral approach that will limit his free will.