I think if we are talking raw semantics here, I would explain the difference between a main idea and a theme by going into more depth on the explanation of "theme."
When asking students to identify the "main idea" of a piece of text, very often it is as simple as identifying the subject of a paragraph, or the most important event or detail of a chapter in a novel.
A theme, however, goes beyond paragraphs and chapters. I explain this literary element to my students using the following list.
- is a central (or main) idea that is prevalent throughout the novel or text.
- is an idea presented by the author that applies to the text but can be brought out of the book and applied to the world as well.
- might teach a lesson, but is not simply "the moral" of the story.
- is a deeper or more complex truth about a subject in the story that can be supported with plot details, BUT
- is not a summary of the plot.
To me, the term "main idea" does not, by nature, span as broadly as the term "theme," and therefore could, in some cases, be a retelling of a portion of the plot.