What is the theme in The Universe in a Nutshell by Stephen William Hawking?

The Theory of Quantum Gravity is the main theme of the book. The Universe in a Nutshell is a work of largely speculative science. Stephen Hawking first explains how the - in some sense - last major problem for physics was set up and how some propose to solve it. The problem is developing a theory of quantum gravity. The theory of quantum gravity would be a theory of everything. It would describe all physical phenomena at every level of nature. It would thus be a final theory, a theory that described the ultimate nature of the universe.