Here it is I will give you a new site about WordPress Themes like this site. The name is wpgpl.com, it is really clear for me that the contents are WordPress Themes Collection in GPL.
As a beginner WordPress user and customizer this site is right for you. You will be able to learn and how to create and modification wordpress themes with these collection. You can edit, modification and everything in that themes for free.
For an example I use Yashfa themes for my own local blog. I can edit the layout, colors, and position as I want to. Themes structure was esay to read and analyze so, as a beginner I could get my own modification with wonderful results. Interesting ;) just grab the Yashfa themes and get your bes shot.