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.