It is important not to confuse a
theme of a literary work with its
subject.
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.
It is up to the readers to explore the theme of a literary work by analyzing characters, plot, and other literary devices.