What are the motifs in Ender's Shadow by Orson Scott Card?

The story is science fiction, and subsequently science must be a major part. The students perform their battle in zero gravity, and yet the levels on the space station have varied amounts of gravity that points to having attained artificial gravity. Centrifugal force alone could not keep the battle room zero gravity with the entry points having full gravity. The story does not explain how this is done, just that it could be a possibility if humans become more aware of how gravity works. The technology may have been copied from the Buggers without the scientific understanding.