Wow, this is very interesting that they didn't go down the LESS/SASS route of file-level "global" variables, but instead chose to bind variables to elements and their descendents themselves.

Coming from LESS this seemed strange at first, but upon reflection it makes a ton of sense, and provides for fantastic flexibility.

Now to really make this useful, we just need mathematical operators for distance values (12px - 2px) and functions like darken() and lighten() on color values... or is that being proposed somewhere too?