It seems simple, but as a literacy specialist there have been numerous occasions I've observed teachers who don't seem clear on the differences of topic, theme and main idea. These foundational literacy concepts span learning standards from kindergarten to high school. If lower grade teachers understand and teach their standards correctly, then when theme enters the scene in upper grades, the differences won't confuse older students.
Secondary students often struggle with the difference between main idea and theme. After observing many English Language Arts classrooms, it became apparent to me that many teachers struggle as well. Knowing the differences before tackling a lesson will ensure teachers are able to guide students in their understanding of each literary element.
Planning tip: Try identifying Topic, Theme, and Main Idea of short selections in teacher planning sessions prior to teaching the lessons to students. Practice as a group, or write short stories that highlight each literary element.