What is the theme in Neverwhere by Neil Gaiman?

Richard's story is about coming of age. In London Above, Richard is a metaphorically a child. He knows neither fear nor worry. Richard's problems, whatever they might be, are not matters of life and death. Jessica, in this metaphor, is like a mother; she tells Richard what to do, what to feel, and how to behave. It isn't until Richard thinks for himself, in defiance of Jessica, that Richard begins to mature.