This is a case where the use of alpha channels in HSL color values
shows its limits.
The use of an alpha channel is necessary for the background
elements for the top navbar elements, because they sit over the
top of the navbar's bottom border (actually an inset shaddow).
However, it's impossible to use the alpha-channel based color on
elements like the unread dot, where the border actually sits on
top of the element itself--meaning that the effect would be a
larger dot with an imperceptibly darker ring around it.
What this commit does is use a technique suggested by Anders
Kaseorg for using CSS's `color-mix()` functional notation to
calculate an opaque version of the alpha color for use on
elements that do not or cannot directly take the color with
the alpha channel.
See CZO discussion:
https://chat.zulip.org/#narrow/stream/6-frontend/topic/alphas.20in.20color.20definitions/near/1670102
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