What is the theme in Rebecca by Daphne du Maurier?

The driving force behind the actions of this book's characters is loyalty. This is seen most clearly in the characters of Frank Crawley, the business manager of Manderley, and Frith, the head butler. Crawley expresses his loyalty by being congenial, never shying away from a topic of conversation and yet never expressing exactly what he thinks either. Mrs. de Winter can sense that Crawley is on her side, but she also knows that he will not be completely honest about what he thinks of Rebecca because his sense of loyalty to Maxim would forbid it. Frith is just as deeply loyal, but it is easier for him to keep up his attitude of detachment because, as a servant, he is not involved in family matters nor expected to know about the de Winters' affairs anyway.