The theme of the book "the catcher in the rye"?

I think Salinger was writing an appreciation of how intensely and idealistically young people sometimes see the world, and how sad it is when kids grow up. Holden wants to protect kids who are even more vulnerable and pure than he is, and that's why he wants to catch them in the rye before they hurt themselves, and that's why he rubs out the vulgar and offensive word, so typical of adults, that he sees on the wall.