For me themes come from a creators core beliefs. A lot of folks I run into mistake theme for character motivation. A THEME is made up of multiple solitary words. A character's motivation is a sentence.
For example:
In Toy Story Woody wants to be Andy's favorite toy.
That's not a theme. That's a motivation.
In Toy Story, the actual story is about friendship. Here FRIENDSHIP is your theme.
More complex stories can have multiple theme's where one is the lead singer and the others are the backup harmonizers. Like in Coco the theme is Family, but it's backed up with Remembrance, Tradition and Love.
A good way to find your theme? Think on what your story is about. Now sum it up in one or two words. For my comic, if someone asks "What is the story of Kamikaze ABOUT?" I'd say: Sacrifice and Perseverance.
Theme's are large, big ideas that can be just about anything that IS NOT A NOUN.
Loyalty.
Faith.
Temperance.
Charity.
Suffering.
Understanding
Cooperation.
Family.
Responsibility.
Love.
Trust.
Imagination.
And that's just a short list. Think about the things you feel really deeply about. Core ideas that get you excited and passionately talking to others on. Good stories SAY something about the world. I know it's really easy to brush that away with, "I mean sometimes it's just a dumb book," but usually if you ask the creator about it? That's not their thoughts at all.
Figure out what you believe in life. Find your theme. Then get to work.