How can one topic in a text be the foundation for several themes?

“Theme is the central message of a literary work. It is not the same as a subject, which can be expressed in a word or two: courage, survival, war, pride, etc. The theme is the idea the author wishes to convey about that subject. It is expressed as a sentence or general statement about life or human nature. A literary work can have more than one theme, and most themes are not directly stated but are implied. The reader must think about all the elements of the work and use them to make inferences, or reasonable guesses, as to which themes seem to be implied.

Explanation:

For example, if love is a topic/subject of two novels, a major theme in one of the novels could be “Love, if taken to extremes, can be negative rather than positive,” while in the other novel, the theme might be “Love can conquer even the greatest evil.” Notice that the topic/subject is the same, but the messages about that topic/subject are different in different works.