According to Web Content Accessibility Guidelines, the minimum contrast ratio between <18px text and its background should be 4.5:1. #999 on #fff (@Gray-Light / ".text-muted" on "body") has a contrast ratio of 2.85:1, lower than recommended even for large text.

#767676 for @gray-light would give a contrast ratio of 4.54:1 and pass for all sizes of text.

I used the WebAIM Color Contrast Checker to test the values.