What are the motifs in The Hitchhiker's Guide to the Galaxy by Douglas Adams?

In a sense, this novel presents the entire universe as belonging to a different culture that citizens of Earth just do not understand. The non-Earth characters, from Vogsphere, Betelgeuse, and Magrathea, all seem to understand each other, even in cases where they have not been introduced to each other's culture before. For example, both Ford Prefect and Zaphod Beeblebrox believe the planet of Magrathea to be a myth, but soon after they arrive they accept it for what it is with little further conflict. Even Trillian, who was born on Earth but has been traveling the universe for roughly six months more than Arthur Dent, is unshaken by the strange occurrences that she observes.