Darken @gray-light to meet accessibility guidelines

`@gray-light` darkens from `#999` to `#777`, which puts `.text-muted` at the
threshold for the 4.5:1 WCAG minimum contrast[1]. `#777`:`#fff` is 4.48:1.

The “Example” headers in docs become #959595, which is contrast ratio
3:1, the minimum for larger text. Since the headers are less important
than the surrounding text, 3:1 is fine and an improvement on the
previous `#bbb`:`#fff` (1.92:1).

Fixes issue #13847.

[1] http://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html