In contemporary literary studies, a theme is a central topic a text treats. Themes can be divided into two categories: a work's thematic concept is what readers "think the work is about" and its thematic statement being "what the work says about the subject". Themes are often distinguished from premises.
this is the link https://en.wikipedia.org/wiki/Theme_(narrative)
Explanation:
Subject is a topic that acts as a foundation for a literary work, while a theme is an opinion expressed on the subject. For example, a writer may choose a subject of war for his story, and the theme may be his personal opinion that war is a curse for humanity.
