Here's why:
* Theme is the underlying idea or message that the author is trying to convey through the story. It's not explicitly stated but rather emerges through the characters, plot, setting, and symbols.
* Central message is a more general term that can refer to any important idea in a work, but theme specifically refers to the core concept the author wants the reader to take away.
For example, in "To Kill a Mockingbird," the theme of racial injustice is woven throughout the story, but it's not explicitly stated. The reader understands it through the characters' experiences and the societal context.
So, while the central message and the theme are related, theme is the more specific term for the overarching idea the author wants to convey.