Theme is the, erm... theme of the story.
The theme is a piece of guidance.
Normally there are more than one theme.
However, there is going to be a major theme.
For instance, a theme for the Harry Potter series might be
<aside> 🤔 Friendship is important.
</aside>
The topic is different from the theme as it is shorter, usually only one word.
A theme, however, is a sentence.
As we all know, there are usually 3 purposes.
Many characters are worth comparing/contrasting.
For instance, friends are likely compared, while enemies are likely contrasted.