I've gotten the basics of Wordpress down. Now I want to learn a little more advanced including the back end PHP, CSS, etc. I want to be able to edit already made themes to make tweaks and eventually be able to create my own themes. I already have a giant PHP book, however I'm looking for a PHP/CSS book or guide or tutorial that teaches me the code with references to Wordpress, and not just PHP/CSS in general. I would appreciate any input.