What is the theme of Mortal Engines by Philip Reeve?
One of the central themes of this post-apocalypse dystopia novel is the destruction of our present way of life and the following means of restoring civilization and technology and reclaiming lost humanity.
[some things] had somehow survived through all the millenia since the Ancients destroyed themselves in that terrible flurry of orbit-to-earth atomics and tailored-virus bombs called the sixty Minute War.
Some things in the restored civilization are different the present day while some thing are the same, including the proclivity to identify any figurine as a deity of the past civilization, even platic figurines of Pluto and Mickey Mouse: "He cut ... past the big plastic statues of Pluto and Mickey, animal-headed gods of lost America." Other things that are the same is the town names, like London, and town functions, like mining town. London still has the Museum and the Bloomsbury district. The rich and famous still have mansion separated from the common people. People still work and have jobs, though in some respects these are more Medieval in nature, like Guildmasters and Apprentice Third Class.
Many things are different however. Cities, towns and sections are elevated a couple of thousand feet above the ground. They are piled up and tiered, "like a wedding cake." They run along the abandoned Earth ground on tractor wheels that run in and create massive tracks in the ground, the Great Hunting Ground. The biggest difference is that elevated cities and towns hunt and devour smaller ones, doing this to the point of scarcity or ecosystem capital depletion (which is something that is not different from the present day).
The theme, therefore, is one of self-destruction through two avenues [(1) resource depletion and (2) vicious military attack], restoration of civilization and the reclamation of humanity, which is always lost during self-destruction. Humanity is reclaimed through the adventures and efforts of the disenfranchised hero and heroine, Tom and Hester, who are sent overboard down a waste chute to the Hunting Ground but who pursue London unrelentingly (from different motives) anyway.