Wordpress Desktop Website

I'm currently looking for a course to learn how to build custom themes—it's a bit embarrassing to admit that I don't know how. I learned to code custom websites with only html and css, so the transition to using website builders has been fairly seamless. However, working with prebuilt themes is like trying to do yoga in jeans—it's constricting and cutting off my creative circulation.
Any recommendations on where I should start?
I worked on this project with a web developer on this wordpress site and my role was purely on the aesthetics.