The key differences between a theme and a central idea is the genre that each pertains to and their purpose. A theme comes from Fictional, poetic, or narrative writing while a central idea relates to nonfiction, historical, or direct accounts. Well both represent the main ideas of a text, what separates them is the genre of the writing. A theme also leans more to teaching a lesson. That is able to be universally applied, while a central idea simply represents the main idea of a text.