By coincidence, I created a new s9y based on the CSS framework Skeleton. It is going to be included in the s9y core starting with the next 2.1 beta, I think. Skeleton is plain & simple, kind of typography-focussed. The theme is HTML5 and responsive, it has a single sidebar (on the right) on screens large enough and uses the core nav. And that's about it already.
Questions you might have:
Is there a demo? Nope. I only have a development blog, and I might need that for something else. There's a preview, for what it's worth.
Can I use this now? Technically, you could get it from the current GitHub master. I would not advise it unless you run a 2.1-beta installation, though. I used this to test some stuff onli and I were working on for the preview, that might or might not work in 2.0.
Can you add $COOL_FEATURE? Probably not. I don't intend to make this a fully-fledged theme with a lot of options; it doesn't even have a dedicated mobile nav.
Can you change this-or-that? Probably not, unless it's a bug (which the Skeleton CSS is supposed to cover if it's a CSS bug). You can, though, using a user.css.