In contemporary literary studies, a theme is the central topic, subject, or concept the author is trying to point out, not to be confused with whatever message, moral, or commentary it may send.
You may recall reading To Kill a Mockingbird where the expressed theme of the novel is coexisting in an imperfect society. Racism is morally wrong is a thesis statement derived from the theme. Novels will certainly convey several themes; the reader should look for the central topic however and attempt to express the understanding of theme universally rather than specific to the character’s conflict. The author generally attempts to convey an understanding from the theme that his/her reader can identify with as to invoke empathy with the protagonist’s struggle.