style(mixins/vendor-prefixes.less): align properties vertically
refactor(transition mixin): use merge operator

LESS's merge operator ('+') is a better use case for the transition
mixin, which may be used more than once in a single ruleset.
refactor(scaffolding): set background in html

The html tag should receive the same background as body, so
if we have some element with z-index: -1 which is positioned
against body, it doesn't show behind other content.