Make spinner colors consistent

[pkwi]

Bug report

• Package version(s): 1.3.1
• Browser and OS versions: all

Actual behavior

dark spinners use the following colors:

Track: gray1 (20%)
Head: gray1 (100%)

Expected behavior

Track: black (30%)
Head: gray1 (100%)

[leebyp]

@pkwi the contrast with the proposed colors are coming out different to your screenshot, do we want to tweak some more?

[pkwi]

@leebyp ha! my fault. The head should be using gray3 (#8A9BA8) not gray 1.

[giladgray]

@pkwi the contrast between the track and background in your proposal is very low, almost too low...
though in the current incarnation the contrast between track and head are too low...

[pkwi]

Valid point @giladgray! We could increase the track's opacity to 50% (same colour/opacity as dark sliders)

[leebyp]

like the 50% a lot more (though from looking at the docs page, maybe we can go even higher)