Dostoyevsky does that thing where every character symbolizes a different answer to a thematic question (or at least a different variation on a basic theme like “love”). His books are very PG-13, but excellent examples of thematic writing taken to the extreme. I recently finished his book The Idiot, in which the main character represents Christian brotherly and compassionate love and various other characters represent passion, self-hatred, romance, and pragmatism.
I don’t know if it’s still open, but Josiah’s Theme Mastery course would be an excellent place to learn the difference between different kinds or qualities of theme (like, “theme” and “message” are two different things, and he gives great advice on both): https://josiahdegraaf.teachable.com/p/theme-mastery
I also highly recommend Invisible Ink by Brian McDonald. It’s an extremely Pixar-ish look at theme, and one of my favorite writing books of all time. He wrote another book called The Golden Theme, which is also good, but it really just expands what you already read in the first and better book.
"...the one with whom he so sought to talk has already interceded for him." -The Master and Margarita