creating a wordpress theme from scratch is actually pretty easy if you know the basics..I've been playing around with existing themes, modifying them like crazy for years, but then I realized that I will never find the perfect theme until I make one myself. since then, i made tube themes, simple blogs, web shops, paysites, tgps etc.. i didn't take any courses, just used google when I was stuck
There's a really simple wordpress theme which i was using for years, called
CP-Minimal. It's simple enough, fast and it basically gives you all the basics you'll need for developing your own themes.