Slides for my talk at WordCamp Toronto 2014. The full theme is available here: https://github.com/LinnAlexandra/wcto14
I updated this talk in 2016 for WordCamp Montréal: https://speakerdeck.com/linnalexandra/dont-fear-the-custom-theme-how-to-build-a-custom-wordpress-theme-with-only-four-files-1