Howdy! This theme, Toolbox, has been retired. If you want to style a theme with CSS from scratch, you can do so with any theme in WP.com—and most of our new themes are an updated version of Toolbox behind the scenes. :)
It’s part of the CSS tool of the Custom Design Upgrade. When editing CSS, click on the “Start Fresh” checkbox. That will give you a blank theme to design with.
For self hosted users, Toolbox has seen an update in the form of a brand new theme, _s.