Theme is the overriding idea of any piece fiction.
Step-by-step explanation:
In a piece of fiction, the theme is always the overriding idea of the whole work as it holds all the elements together and does not let the elements disperse. The mood is created using the choice of words, with correct words the mood can be set to be serious or even joyful.
It evolves naturally from the characters as well as the plot. We can also assume that the theme is the hidden or the underlying message of the story that the author wants to convey to the readers of the story.