The theme for "The Djinn Who Lives Between Night and Day" would be not to always believe what people tell you.
It applies to the story because Al-faq the djinn went to the husband and said to him "Get up," said Al-faq. "Go out. Walk south. Walk until you can go no farther. I will give you a sign that your wife and children have returned to life, but you must stay there by the sea, alone, with nothing. You must never speak again. You must never seek the ones you love, for if you find one, then all eight must die." And the man believed him! He ended up walking all the way to the sea.
This theme also applies to real life because you shouldn't believe everything that you hear, for example if someone walked up to you and said that there was no school tomorrow, and if they were lying then you would miss school and get into trouble! So you see how this theme relates to life.