What doest theme mean?

i always thought that theme meant a moral. like "do your best". or something. but my sister said it wasn't. for example, there is a book called "the curious incident of the dog in the nighttime. its about an autistic kid trying to make emotions logical. and i thought the theme was "accept everyone for who they are". but my sister says that its "the search for order and stability" . i dont have a clue how thats the theme. plz help.