It depends on which version of the book you are referring to. Anthony Burgess published two versions of A Clockwork Orange. His first publication was a European publication and included a chapter 21. The U.S. version did not include chapter 21, and the film is based on the U.S. version of the book. In fact, Kubrick claimed that he didn't even know there was a version with a 21st chapter. That missing chapter makes all the difference for a theme comparison between book and film.
Some readers (of the 20 chapter version) and viewers of the film believe the central theme and message is the glorification of sex and violence. Those things are definitely in both the book and the movie versions. In fact, the book versions might even be more horrific than the film. For example, in the movie Alex has consensual sex with two teenage girls. In the book, he rapes two ten year old girls after getting them drunk. There are other graphic scenes in the books that Kubrick didn't put into the movie either.
On the other hand, it is the central notion of Alex's reconditioning and eventual character change that the movie portrays very differently than the 21 chapter book. In the movie Alex's conditioning is forced upon him and he is conditioned against only Beethoven's Ninth. In the book, Alex volunteers for the treatment. He's also conditioned against all classical music.
Maybe readers and viewers don't think those differences are major. But the film did NOT include chapter 21. In that chapter, the reader is told that the conditioning worked. Alex is reformed. He is a changed man. He grows out of his taste for violence. He is looking forward to a life with a family. A wife. A child. When that version of the book is read, the reader sees a theme of redemption. A reader sees hope for Alex and/or mankind and sees that it is possible for a person to take active steps to set their life on the right path. The movie ends in the opposite manner. Alex is deconditioned and the viewer is left with the knowledge that Alex is going to revert to his violent, sexual promiscuity again. That ending is dark and depressing, and leaves viewers with a feeling that there is no point in trying to rehabilitate and redeem people.