By a theme we mean an idea that runs through a text. A text [text: Any piece of writing. More widely, a text can be anything that conveys meaning - eg, a film, tv programme, advert, website, or image. ] may have one theme or many. Understanding these makes the text more than 'just' a text - it becomes something more significant because we are encouraged to think more deeply about the story; about how it connects to real-life issues, and what it might mean to us.
In Chapter 10, Jekyll writes clearly about the dual nature of human beings. He says that, as a young, educated man from a respectable family, he maintained an appearance of good behaviour at all times. But he says this was a fraud - no one suspected his true nature, which was at times extremely immoral.
Jekyll's experiments began in an attempt to separate the two sides of human nature and destroy the evil one. He discovered that the evil part of his nature was, indeed, part of himself, and therefore, in some sense, natural and part of the whole.