Add colors to theme: white opacity 15% and black opacity 15%

[adamwoodnz]

Used for the top border of the updated local header in Learn sites:

[ryelle]

In showcase, I added the border as a custom color #ffffff0a, rather than creating a new color for this. I was on the fence about whether it's necessary— I'd rather not start adding color+opacity combinations, it could make the palette selector unwieldy.

@WordPress/meta-design Will these two be the only transparent colors used on the site? I'm not sure if this color list is from the previous redesign iteration. If it is just these two, then I think adding them would be fine.

[jasmussen]

Will these two be the only transparent colors used on the site? I'm not sure if this color list is from the previous redesign iteration. If it is just these two, then I think adding them would be fine.

I think we can decide that, yes.

[adamwoodnz]

👍 I'll update this issue and PR to add both 15% opacities