The dominant theme, I think, is censorship. Books, in particular, are censored in the future. It is never exactly clear why this has happened, but it seems to have occurred in the context of the rise of television, as post #2 suggests, and in a society that is dominated by interest groups that feel the need to censor things that offend them or are deemed against their interests. This theme reaches its logical (or illogical, as Bradbury is trying to show) conclusion in the section entitled "Burning Bright."
I would suggest that the theme is the same as one of the ones for the book. Television will be the decline of civilization! All kidding aside, he watches the television, which causes the decline of civilzation between people don't talk to each other.