My experience themeing Drupal, like most of my coding skills, have been developed by digging up useful resources online and some trail and error. I have an interest in graphic design, but never really studied it. I can turn out sites which look good, but my “designs” don’t have the polish of a professionally designed site. I own quite a few (dead tree) books on development and project management. Generally I like to read when I am sick of sitting in front of a screen.