What is the theme in The Myth of Sisyphus and Other Essays by Albert Camus?

The chief theme of the entire collection of essays is that the world has no meaning and cannot be understood. Human reason naturally wants to try to understand things in human terms and find a unity in the world. Many attempts have been made to do this, but all such attempts ultimately fall apart. This is because, unlike human reason, the world is fragmentary and distinctively inhuman. As a result, reality can never truly be penetrated and any truth—other than that truth is impossible—can never be attained. Absurdity it should be noted is not a feature of the world itself, but rather the relationship between the mind and the world. Absurdity, in general, is a lack of proportion between two things. Therefore, absurdity is the lack of proportion between the unifying human mind and the non-unified and irrational world.