According to Bradbury himself, one of the important ideas treated in Fahrenheit 451, is the idea of catering to special interests, of trying to please everyone at all times.
Eliminating the risk of offending anyone, of presenting ideas that may be contrary, as revealed in the novel, leads to the kind of society seen in the novel. Ideas need to be presented whether or not they will offend anyone, and whether or not everyone will agree with them.
Bradbury explains that at the time he wrote the novel he was constantly receiving complaints from group after group who disagreed with things in his works, and would even receive requests to use his works with certain things edited out, so that they would not offend or present contrary ideas. He refused these requests.
The Captain outlines this theme in his words with Montag, and the theme is made concrete in Millie's reactions to what Montag reads, and in the reactions of Millie's friends when Montag reads. Anything that is contrary to their self-assured, entertainment-filled, trivial existence is anathema. They want to be left alone--they do not want to be made to think. They want to hear only that which they agree with.
In my opinion, the overall theme of this book is that people can lose their freedoms and even their ability to truly be human if they do not work to protect those things. In the book, the people of the society have lost their ability (most of them, at least) to think and to care about each other and they have lost their freedom to do certain things (like read books).
What's important is that it wasn't some evil government that took away these abilities. Instead, the people stopped wanting them. In fact, the people demanded that they be taken away because those things made people uncomfortable. This is a case where the people themselves decided that they valued security more than they valued their freedom.
So the idea behind the book is that people will give up their freedoms and their humanity in favor of predictable lives where they are not challenged to think or to deal with problems. Bradbury is telling the readers to be careful so that this does not happen to their own society.
In my opinion, the theme of Farenheit 451 is that human society needs to slow down. In Bradbury's world, people have become so occupied with thrills, mindless entertainment, and bright colors that it pains them to think. It pains them so much, in fact, that books have been outlawed by the government. This move by the government was not the government's idea. Rather, they merely had to further a process that had already been begun by society. That shows how careless people can get, and that society needs to be more careful about what they are become de-sensitized to.