Let me help you!
The main idea or theme of the story, "The Girl Who Drank the Moon" written by Kelly Barnhill, is standing up ffor someone dear to you even if it means letting go of your comfort zone. It also talks about blind allegeiance and oppression.
The best part is probably the part where the mothers of the protectorate begin to remember the children they sacrificed to the witch, because you realize that the people making the decisions in the Protectorate are mostly men.