The theme in a piece of writing is an important idea or subject that runs through a text. Themes invite us to consider what the author is trying to say about a certain aspect of life and help the text become more significant to us because we are encouraged to think more deeply about the story.
A text may have one theme or many. The themes we study in Jekyll and Hyde concern binary opposites, that is, complete opposites like wet and dry or hot and cold. This is most obvious in the main theme of duality.