@ashlyvye Great question. It can be confusing because the word has a wide variety of uses in normal English, but in the writing world it has a very specific definition.
What you’re thinking of is genres. When writers use the term theme, they mean something different.
Here’s an article I wrote that explains what theme is and gives some basic tips for how to apply it in your writing. http://writershelpingwriters.net/2018/02/theme-made-simple/