Central Themes vs. Central Ideas
Teachers and writers sometimes use the terms "central theme" and "central idea" interchangeably, because they both refer to basically the same concept -- the basic premise or message of a text. However, "theme" typically connotes a literary work, while "idea" more commonly refers to nonfiction or informational texts. Main themes and ideas dominate and drive an entire text; they are not limited to one chapter, sentence or character.
1 Central Themes
All stories present a particular view on a topic, whether implied or stated. Often, the protagonist develops this view or perspective as the story progresses. For example, in the fairy tale "Beauty and the Beast," the heroine Belle learns that a person's appearance does not necessarily reflect his character, as evinced by Gaston, who is beautiful outside, but ugly -- arrogant and selfish -- inside, while the Beast is ugly outside but tender and compassionate inside.
2 Central Ideas
Nonfiction and factual texts, ranging from business reports to advertising, operate under certain premises as well. Marketers try hard to incorporate value messages in their advertisements, such as the idea that buying trendy clothes can improve your self-image, which may or may not be true. Marketing messages are often only implied, while more formal writing, such as essays, usually directly state their central ideas for clarity and efficiency. A busy executive needs to grasp the central thrust of a work memo or progress report immediately. Central ideas in professional writing tend to deal more with specific, pragmatic topics, rather than the abstract, meaning-of-life questions found in literary works.