What is the theme in The Amber Spyglass by Philip Pullman?

Technology and evolution is a major theme. The Mufela do not have a center spine and they roll rather than walk, using the pods from the seed-pod trees as wheels when they come to a certain age. In Lyra's world, there are gyropters and zeppelins rather than helicopters and jets. The Chevalier communicates with a lodestone resonator. Technology and evolution are "what if" themes that can be freely explored in fantasy/science fiction/speculative fiction.