What is the theme in Neverwhere by Neil Gaiman?

The contract is another theme in the story. When Richard first meets the marquis de Carabas, he tells him that Door offers "nothing" in exchange for his services, only mentioning the "really big favor" as an afterthought. This demonstrates the difference in thinking between the two Londons. To Richard, a mere favor seems almost without value. To the marquis, however, a promise is as good as gold. This suggests that, in London Below at least, something compels such obligations to be met.