From https://thecreative.cafe/finding-themes-in-your-writing-fc4959113c83:

What is Theme? Definition, Examples of Theme in Literature

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Definition of theme: The theme of a literary work is a salient abstract idea that emerges from the treatment of its subject matter. Common themes are love, war, deceit, revenge, fate, destiny, etc.

What Does Theme Mean in Literature?

What is a theme in literature? A theme is a message or abstract idea that emerges from a literary work’s treatment of its subject matter.

The theme differs from the subject itself. The subject of a work can be described in concrete terms, usually through actions.

For example,

• The Adventures of Huckleberry Finn.

The subject matter is Huckleberry Finn’s adventures. The theme, however, is more of an abstract idea. The primary theme of Huck Finn, for example, is the conflict between civilization and natural life.

Theme vs. Subject

The theme of literary work deals with an abstract idea or concept, while the subject deals with the concrete actions that bring us that idea.

Example of Theme:

• Theme = Coming of Age.
• Subject Matter = A newcomers difficulties in moving from a small town to the big city.
• Explanation: The theme coming of age will explore the growing pains adolescents are likely to experience in the world. The subject matter for our hypothetical literary work is someone who recently moved from a small town to “the big city.”

Example of Theme:

• Theme = Pride.
• Subject Matter = The struggles of varsity football players to work together.
• Thematic Statement = It is important to maintain a balance between pride and humbleness because too much self-confidence can be destructive.
• Explanation = In this example, we see the author’s opinion regarding the theme of pride and how it relates to the work’s subject matter.

The thematic statement of a work is rarely said explicitly. Instead, people interpret it through characters, actions, and events within a book.

Thematic Concept vs. Thematic Statement

The thematic concept of a work is basically the definition of theme that we have outlined above. It is the abstract idea or concept with which a work of literature interacts.

A thematic statement, however, can be read to be more of an argument about that concept. In an essay, a thematic statement would be called your thesis statement.

For example,

• Theme / Thematic Concept = Freedom.
• Thematic Statement = Freedom should be extended to all citizens of the world.

As you can see, the theme of freedom is abstract and broad, and it doesn’t have a thesis statement. The thematic statement of a work, however, will make a claim about this concept. Think of the thematic statement as being an opinion held by the author about the theme itself.

In our example above, the theme is freedom, and the thematic statement is freedom should be extended to all citizens of the world.

There are also central themes and minor themes in literary works, which are just as they sound.

• Central theme = the central or main message of a literary work.
• Minor themes = other, less important messages of a literary work.

Examples of Theme in Literature

Here are some examples of themes found in literature:

• In Paulo Choelo’s novel, The Alchemist, the author explores the subject of fate through a person’s dreams. His thematic statement regarding dreams is that one should always follow his heart and pursue his dreams at any cost.
• In William Blake’s poem, The Poison Tree, the author explores the theme of anger. The thematic statement of the poem is if anger is not dealt with, the feeling will intensify and eventually leads to destruction.

Summary

Define theme in literature: In summation, the theme is an idea or concept that a literary work explores: love, despair, honor, etc.

The author explores the theme through the subject matter of his or her work, and when telling the story, arrives at some kind of thematic statement.

Here is a final example of theme found in the short story “The Gift of the Magi” by O. Henry.

In “The Gift of the Magi,” Henry conveys the message that love is more important than material possessions.

• Theme: Love, charity, sacrifice.
• Subject matter: Young couple struggling financially during Christmas.
• Thematic Statement: Love is more important than material possessions.

From https://thecreative.cafe/finding-themes-in-your-writing-fc4959113c83:

Hiding in plain sight are ideas or beliefs that underpin some if not all of my work.

Theme in writing

The theme of a story is what the author is trying to convey — in other words, the central idea of the story. Short stories often have just one theme, whereas novels usually have multiple themes. The theme of a story is woven all the way through the story, and the characters’ actions, interactions, and motivations all reflect the story’s theme.
Cliff’s notes

In well written stories, theme gives a satisfying sense of ‘I know what that was all about’ in terms of universal ideas like love conquers all or family comes first. Theme is separate to plot or what happens and where. Love can conquer all in any number of different settings.

Conversely, a story without a theme, even if well written with engaging characters, leaves the reader wondering ‘so what?’ And a story written to a specific theme can come over as preachy, especially when political or religious. The reader feels they’ve been beaten over the head with a blunt instrument.

Discovering your values

Most of us don’t examine our core beliefs on a regular basis, if ever. As writers hoping to illuminate the human condition through stories, it might be useful to our selves and to our readers to dig a little deeper into the beliefs that drive our behaviour. We might ask ourselves questions like

• what makes me happy
• is my life the result of luck or choice
• what is the strongest emotion
• is ‘blood thicker than water’
• are people essentially good or essentially sinful
• are rules made to be broken

Looking a little deeper at the answers will help you understand yourself and what guides your choices. And of course in fiction, you can use values to build a compelling character who behaves like a real person in the story. A list of useful questions to ask and a summary of values can be found here at mindtools.com.

Using theme to your advantage

Sometimes the theme is only seen when looking back at a work. During the first draft our job is to tell ourselves the story, as Terry Pratchett said. After a break, re-reading the story should reveal its point, if you didn’t write with one in mind. It might be something unexpected. Your job then is to strengthen the theme using subtle hints in characterisation and dialogue, so that the narrative hangs together. On the other hand, if theme is too obvious, it may need toning down so that it fades into the background.

If genre is an ocean and plot is the wind steering the boat of characters, they can change course by their actions. But the theme is like a deep ocean current that will bring them to a particular destination, even though they can’t see it.

New directions

I don’t want to keep writing the same story. So periodically I have a look at my values again. They change in importance and evolve as my life does, and my art should reflect that. And that is one of my personal values.

If we don’t change, we don’t grow. If we don’t grow, we aren’t really living.
Gail Sheehy

Look at your works, or better yet ask someone else to read them, and see if a recurring idea or value reveals itself. You may be surprised. Then have a go at a new piece, keeping your theme in mind.

Let theme inspire you.

From https://www.thoughtco.com/common-book-themes-1857647:

10 of the Most Common Book Themes

While there are countless themes in books, a few are the most common. These universal themes are popular among authors and readers alike because they are experiences we can relate to.

To give you some ideas on finding a book’s theme, explore some of the most popular and discover examples of those themes in well-known writings. Remember, however, that the messages in any piece of literature can go much deeper than this, but it will at least give you a good starting point.

1. Judgment: Possibly one of the most common themes is judgment. In these books, a character is judged for being different or doing wrong, whether the infraction is real or just perceived as wrongdoing by others. Among classic novels, you can see this in “The Scarlet Letter,” “The Hunchback of Notre Dame,” and “To Kill a Mockingbird.” As these tales prove, judgment does not always equal justice.
2. Survival: There is something captivating about a good survival story, one in which the main characters must overcome countless odds just to live another day. Almost any book by Jack London falls into this category because his characters often battle nature. “Lord of the Flies” is another in which life and death are important parts of the story. Michael Crichton‘s “Congo” and “Jurassic Park” certainly follow this theme.
3. Peace and war: The contradiction between peace and war is a popular topic for authors. Quite often, the characters are gripped in the turmoil of conflict while hoping for days of peace to come or reminiscing about the good life before the war. Books such as “Gone With the Wind” show the before, during, and after of war, while others focus on the time of war itself. Just a few examples include “All Quiet on the Western Front,” “The Boy in the Striped Pajamas,” and “For Whom the Bell Tolls” by Ernest Hemingway.
4. Love: The universal truth of love is a very common theme in literature, and you will find countless examples of it. They go beyond those sultry romance novels, too. Sometimes, it is even intertwined with other themes. Think of books like Jane Austen‘s “Pride and Prejudice” or Emily Bronte‘s “Wuthering Heights.” For a modern example, just look at Stephenie Meyer’s “Twilight” series.
5. Heroism: Whether it is false heroism or true heroic acts, you will often find conflicting values in books with this theme. We see it quite often in classical literature from the Greeks, with Homer‘s “The Odyssey” serving as a perfect example. You can also find it in more recent stories such as “The Three Musketeers” and “The Hobbit.” 
6. Good and evil: The coexistence of good and evil is another popular theme. It is often found alongside many of these other themes such as war, judgment, and even love. Books such as the “Harry Potter” and “Lord of the Rings” series use this as the central theme. Another classic example is “The Lion, the Witch and the Wardrobe.”
7. Circle of life: The notion that life begins with birth and ends with death is nothing new to authors—many incorporate this into the themes of their books. Some may explore immortality such as in “The Picture of Dorian Gray.” Others, such as Leo Tolstoy‘s “The Death of Ivan Ilych,” shock a character into realizing that death is inevitable. In a story like F. Scott Fitzgerald‘s “The Curious Case of Benjamin Button,” the circle of life theme is turned completely upside down.
8. Suffering: There is physical suffering and internal suffering, and both are popular themes, often intertwined with others. A book such as Fyodor Dostoevsky’s “Crime and Punishment” is filled with suffering as well as guilt. One like Charles Dickens‘ “Oliver Twist” looks more at the physical suffering of impoverished children, though there is plenty of both. 
9. Deception: This theme can also take on many faces. Deception can be physical or social and it’s all about keeping secrets from others. For instance, we see many lies in “The Adventures of Huckleberry Finn,” and many of William Shakespeare‘s plays are centered on deception at some level. Any mystery novel has some sort of deception as well.
10. Coming of age: Growing up is not easy, which is why so many books rely on a “coming of age” theme. This is one in which children or young adults mature through various events and learn valuable life lessons in the process. Books such as “The Outsiders” and “The Catcher in the Rye” use this theme very well.

A huge list of common themes:

https://literarydevices.net/a-huge-list-of-common-themes/