The most noticeable way an author develops a theme is through key words.
For example, if I wanted to make a darker tone, I'd say "emotionless" rather than "content", or "mortified", rather than "nervous."
Also, authors tend to add small details that they want the reader to notice. Such as dramatic effects, like the reader knows something that the narrator or main character doesn't. This could also mean describing scenes or props with a greater amount of detail than expected.
I hope this helps! :)
