What is the theme in The Hitchhiker's Guide to the Galaxy by Douglas Adams?

One of the guiding principles of The Hitchhiker's Guide to the Galaxy is that of absurdity, of things happening randomly without cause or meaning. This does not mean that the whole book is a series of events that occur in random order. Most of the extreme examples of meaninglessness, in fact, do have a cause—they are the products of the Infinite Improbability Drive on the Starship Heart of Gold. The fairly logical explanation of the Improbability Drive in Chapter 10 allows the novel to introduce its most fantastic oddities and coincidences.