They live on Venus, where all it ever does is rain. Each and everyday it pours, nonstop. Margot has seen the sun before because she once lived on Earth, unlike the other kids in her classroom. She believes that the sun is going to come out, but nobody believes her. They are all bullies towards her because she is different, never happy, always talks about the sun.
A boy named William and a few other children bully Margot for being so different. William got the idea of locking her up in the closet, on the day the Sun a supposedly supposed to come out according to Margot.
Later on, the sun came out. Margot was still locked it the closet, while the other children were outside on their only day of sun.
I'm the Green-Eyed Monster!
Once they started to go inside, they remembered. Margot was all alone in the closet, she never got to experience the sun.
They finally let her out.
Jealousy. That is the theme for this story, because they bullied her for being different but really they were upset that she experienced the sun before and they didn't.