I'm really about to date myself here, but this reminds me of Alex King's stylesheet competiton for WordPress 1.2 from the pre-theme days. Swap out the stylesheet and give the blog an entirely new look. (I used Minz Meyer's "Mars Spirit" layout for a few months back in those heady 1.2 days.) While the theme building here is more design and less code, the swappable json of the block themes here and in the previous post make me think we've come full circle, in a way, to the universality of early WordPress where all you needed was a new stylesheet.