Firstly, it should be: it really sets, not set (third person singular - he/ she/ it sets) This means that the feeling/ mood of something has been established. For example: Everyone was wearing designer clothes, which really set the tone of the party. My boss was in a terrible mood and was shouting at everyone, which set the tone for the working day.

"Sets a positive tone" in this context means, creating a positive vibe. By listing down things that you're grateful for, you are starting the day with a positive outlook.

creates an amicable environment for everyone.