Because this text is often considered both absurd and existential, the mood and tone are important considerations. First, in part 1, we see Meursault as almost completely indifferent. He is unmoved by his mother's death; he is unmoved by Marie's need for love; he is unmoved by Raymond's and Salamano's cruelty; quite simply, he is unmoved by the goings on around him. As a result, the mood is one of apathy or indifference.
Tone is also very much lacking in the beginning of the text and much of the way through part 2. If tone is ever changed, it would have to be at the end when Meursault begins to understand the error in his ways. Then, the tone is a bit more harsh and the mood is one of frustration.