The three major themes in Rebecca are loyalty, flesh versus spirit, and guilt and innocence.

Loyalty

Loyalty is seen most clearly in the characters of Frank Crawley, the business manager of Manderlay, and Frith. 

Flesh versus Spirit

The narrator's sense of inferiority results from the fact that she is competing with the memory of a dead woman.  There are several reasons she feels she cannot compete with Rebecca, such as her beauty and social grace.  The ironic thing is that the ghost of Rebecca that haunts Manderley is more a result of terror than grief.  Maxim De Winter remembers her as a mean-spirited woman who put on a false sweet image before the public. 

Guilt versus Innocence

Maxim struggles with guilt feelings for killing her, not because he misses her at all.  One of the author's greatest achievements in this novel is to convince readers of the innocence of the murderer and the guilt of the murder victim.  According to the novel's moral structure, there are several reasons Rebecca deserved to die.  Rebecca was cruel, a liar, and promiscuous.  She threatens to have another man's child and tell everyone that it is Maxim's so that the child would be raised bearing his name.  At the end of the novel readers are left with the impression that Rebecca is guilty and that Maxim, who killed her, is a victim of circumstance.