What is the theme of the novel?
The central themes involved in this story are arranged around the main character, Aerin. She is searching for identity and truth, learning who she and her people are, and coming to terms with the talents, skills and attributes she possesses, along with the fact that she is different from the other people of Damar. Because she has northern blood, there is considerable discrimination against her, and how she deals with this also becomes a major theme of the book. You could also say major themes are perseverance and overcoming personal obstacles and difficulties.
If you are asking "What is theme in general?" it would be the lesson that the reader can take away from the novel. For instance, if one was talking about Lord of the Flies, one might say a theme of that novel is that without strong authority figures or a system of government, people will fall back on their animal instincts to survive.
If you are asking about a specific text, I would need to know which text you are asking about.