Used vh unit to fix ui to viewport height

@commuterjoy javascript hack was preventing me from sleeping, and knew it was possible in pure css. Welcome to the CSS3 vh unit, 1 vh unit is a relative unit of 1/100th of the current viewport height, (vw also exists).

Only downside to this is browser support: http://caniuse.com/#feat=viewport-units

So could add a quick test for support and use JS fallback if not supported. I'll do that tongiht. Should do real work now.

/cc @stephanfowler