When a book "has a theme", the list of options is always short.

(Note, when I reference the word theme, I'm not referring to themes as you probably know them from English class, where they have to be a full sentence directed toward the reader telling them something e.g. "Love is fickle" for A Midsummer Night's Dream. I'm using the colloquial definition.)

The themes could refer to something in history. Some popular ones are colonialism, discrimination, religion, authoritarianism, capitalism, and class struggles.

The themes could refer to something in psychology: Trauma, grief, abuse, consciousness

The themes could be more general: Friendship, perseverance, family

There are several other ones, but most fit in these general categories. But why can't we be a bit bolder with our messages? We could diversify them. There's a whole world of possible messages you could be trying to instill in your readers.

You could have your theme be resource management. You could discuss what it means to manage limited resources and how balancing what's available, what's needed, and what's wanted is important.

You could have your theme be probability. You could have the goal of explaining to your readers how to avoid common fallacies like the gambler's fallacy, what it means for things to be random, and to recognize that many things in life just come down to chance.

You could have themes of analysis. You could have your story tell a message about gathering all the information that is available, processing that information, and remembering things.

If we're willing to go more specific, we could have even more themes. Often, trauma and recovery are themes of many works. But those terms are relatively obscure areas of psychology and sociology that have just been pulled into the public consciousness due to media often featuring them as themes. What if we tried to increase public knowledge about depth and perspective in paintings by featuring them as themes? Or create works about memory management in programming languages? Or, instead of only drawing attention to social issues in politics, explain international trade concepts through literature?

I'll acknowledge that many of these ideas feel foreign, ridiculous, or total stretches to make a whole story about. But there's a vast world of possible messages out there that could be sent. Why restrict ourselves to only a few?