Darken @gray-light to meet accessibility guidelines

BS3 commits: 8c403dc1dcccc38057aba85c49a314a36a93c0af
among supported WebKit/Blink browsers, this now only applies to Safari

BS3 commits: d75506fbec6a50d13802a46b6a1e1fe6bd8655e6
grunt dist

BS3 commits: fdf174e498d8fbd9a73c45ccd805099a25e790f9