A theme is the underlying message or big idea in your story. There are also minor themes that can be in your story as well.
The theme can sometimes follow the wants/needs of the character, it could be the plot that gets realized at the climax, or any conflicts or subplots.
If you follow the Save the Cat Writes a Novel Beat Sheet, Beat 2 is Theme Stated where someone who may or may not be close to your Main Character, tells them the theme of the story, the lesson they're going to learn, their need as a character. The MC won't understand it and will reject it as false- until they learn it on their own at the end of the book.
List of Themes:
Abuse of Power
Acceptance
Anger
Anxiety
Beauty
Change vs Tradition
Circle of life
Coming of Age
Compassion
Conspiracy
Courage & Heroism
Curiosity
Dark and light
Death
Equality
Failure
Faith
Family
Fate vs Free Will
Fear
Freedom
Friendship
Future
Generosity
Good vs Evil
Greed
Growing Up
Hard Work
Hope
Humor
Identity Crisis
Illusion of Power
Immortality
Individual vs Society
Inner vs Outer Self
Joy
Knowledge vs Innocence
Loneliness
Loss
Loss of innocence
Love
Loyalty
Man vs Machine
Man vs Nature
Man vs Society
Peer Pressure
Power & Corruption
Prejudice
Religion
School/Learning
Self-Control
Simplicity
Survival
Taking a Stand
Youth
War
There are a ton more, but I see these most often.
Did any of these spark ideas?
Do any of them repeat in your stories?
I love to use Friendship and Acceptance, with sprinkles of Family, Humor, and Love.
Have fun writing with themes! Don't miss out on the List of Tropes and List of Subplots.