What is the theme in The Baron in the Trees by Italo Calvino?

Living in pursuit of one's ideals is a theme in the story. Cosimo sees as a very young boy that the priorities that shape the actions of his parents are not what he wants to shape his own life. His father chases ideals which no longer exist, and his mother escapes from her world by commemorating battles long-since over. His initial leaving is prompted by one rather insignificant protest, but his staying becomes a way of demonstrating to himself and the world that there are different ways of doing things that might even be better than the one imposed on groups of people by other groups of people. The resolve to guide his own destiny, and the character of a good and selfless member of the community is all Cosimo started out with, and the ideas that shape his political ideals and vision come later as a result of his study.