I wouldn't call it the central theme of the Catcher in the Rye, that's just where the name comes from. The main character of the novel, when asked what his dream job is, pictures himself as a 'catcher in a rye field', catching children from falling off the innocence cliff--into the dredges of adulthood and into the plights of a superficial morally corrupt society. He comes to this idea after being ostensibly homeless, meandering through the streets while trying to find a 'connection' with "real people" similar to how Diogenes did. And being left with nothing but realizing his sister is the only genuine connection he had and he wanted to protect this innocence because of it. Because men are treated like shit and disposables by people once they get into society.
But mainly, the novel deals with male alienation, ostraization, and the loss of identity and follows a disaffected young man, tossed between schools, a delinquent, grounded in nothing, along his existential journey of trying to find a greater purpose.