GlButton: Consider using a lowered opacity background color for tertiary button hover/focus states

Problem

We are using a hard gray for hovering tertiary buttons. However, with a different background, we run into accessibility issues, or at the very least, very harsh color clashing:

Solution

Use a rgba(0, 0, 0, .05) background color on hover for tertiary buttons. This will play much nicer with any background color, simply "inheriting" the background color and darkening it slightly. I think this may have been the intention by using a darkened gray (#fafafa) for use on white backgrounds, but with our dark theme in development and having instances with more than just white backgrounds (like in the above example), it seems to make more sense to use a lowered opacity background to inherit whatever background is there and darken it slightly.

cc @jeldergl This came up when linking to a note recently. I'd like to take a harder look at this after holidays, but feel like we stand to benefit in a multitude of ways by using a darkened, lowered opacity background for tertiary hover and focus states. WDYT?