I really like this book:
https://www.amazon.com/Writing-Your-Storys-Theme-Plotting-ebook/dp/B08L4335LT/ref=mp_s_a_1_2?crid=TGS8DI8REIVP&keywords=theme+km+weiland&qid=1658989310&sprefix=theme+km+weiland%2Caps%2C148&sr=8-2
And here's an article she wrote about it that isn't as in depth, but still pretty good (and free to read):
https://www.helpingwritersbecomeauthors.com/deepening-your-storys-theme-with-the-thematic-square/