The docs site only offered Material's light scheme. This adds the dark
scheme and makes it the default.
- palette: two entries, slate first, each with a toggle. Material takes the
first entry as the default, so the site opens dark and a sun icon in the
header switches to light (and a moon switches back).
- dc-brand.css: the dark scheme's greys are derived from a hue variable, so
it is pointed at the brand slate (206deg) rather than Material's neutral
225deg, which gives the whole scheme the same cast as the header.
- dc-brand.css: the brand slate is unreadable on a dark background, so the
dark scheme uses a light tint of it (#9fb5c6) for body links. Green stays
the accent, which keeps link hover visible in both schemes.
Contrast, over the dark background rgb(30, 36, 41): body text 8.8:1, body
links 7.4:1, inline code 7.2:1, accent green 5.6:1, header text 10.2:1. The
light scheme is unchanged.
dc
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