This is a deceptively simple question. A theme, in short, is the main idea of a story. However, you have to keep in mind that stories, especially good ones have many themes, and some of the best stories have two or more themes that may even compete with each other. In this way, it leaves the reader to decide for himself or herself what the theme is. Let me give you an example.
In the tragedy, Antigone, there are many themes. On the one hand, loyalty to family is a theme. This is why Antigone wants to bury her brother even at the expense of the law of Creon. Creon, the ruler of Thebes does not want anyone to buy Polynices, the brother of Antigone, because he attacked the city.
On the other hand, there is a theme of obeying the laws of the land. So, what should a person do? This is not an easy question. So, here is an example of many themes and in this situation there are two themes that oppose each other.