A theme is a set of ideals that permeate a writing work to prove a point or to highlight a common issue. When a research paper is being written, the writer must cultivate and explain a common issue, depending on the subject he is writing about, such as freedom of speech.
The purpose of a research paper is to draw a light upon a subject and its subsequent impact. The theme gives the paper a backbone or outline for the writer to follow so that the impact can be adequately described and illustrated as important for the reader.
Sometimes a theme can be hidden within the text, allowing the research paper to interpret the overall meaning and importance of a subject.