In contemporary literature, the theme of a text would be The topic of the text.
It can be divided between what the reader thinks the text is about, and what the actual text says about a subject. It is the central idea that supports the approach of the text and the action developed in it. The theme of a text can be resumed in a few words, such as "solitude", "love", "human conflict with nature", but the author's message would be a thesis on that theme, what his or her words imply on that theme.
Also, bear in mind that even if one theme is central, a text can have more than one themes and also express different points of view on them.