In the Monkey's paw the theme is to not wish for something you have enough of because it may not all ways be granted in the good way you want it to be. In the Monkeys Paw ! Mr.White made a wish with the monkeys paw and the wish was to get 200 pounds and the next day his son died and Mr.White got the money he wanted but lost his son.
The authors message is to not ask for something you already have . But you might get it but you will lose something that more valuable and important. For example in the story the man asked for money and then the next day his son died but he also got the money.
The thesis is that you should be specific with what you wish for because stuff doesn't always come the way they seem to be. In the monkeys paw