So what we read on the topic is to use (as base color) the gray which is
perceptually half-way between black and white. Unfortunately it can mean
different colors. For instance if using the sRGB TRC, the #808080 color is
sometimes cited.
Instead I went with the color LCH (50, 0, 0), i.e. with half perceptual
lightness in "CIE LCH(ab) float" babl space, i.e. #777777 in sRGB. It
corresponds to the 18.42% gray which is often refered to, so it seemed a fine
choice.
This replaces proposal in !683 because the contributor is not responding
anymore, and also it uses the common theme base CSS files used by all other
official themes.
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