Give every landscape a second and third hue

Give every landscape a second and third hue

A mark was one palette hue in shades of itself, so a field of avatars was
seven colours in seven degrees of darkness. That is not enough for an
avatar to tell people apart.

The sky is still the account's tone, but the hills now take other tones
from the same seven, chosen from a name-seeded shuffle and never the
sky's own hue. Each hill is shaded toward the palette's darkest ground
rather than toward black, which darkens it and ties different hues into
one landscape. Every mark therefore carries at least two hues, and most
carry three plus the disc.

The seven grounds moved to :root so the bundle reads the palette from
vole.css instead of carrying a second copy of it. A tone class now just
points at one.

Colour sets go from 21 per theme to 7 skies x (30 two-hill orders + 120
three-hill orders) x 3 discs = 3,150, before ridge, radius and disc
position vary them further. keenan, which was clay and four shades of
clay, is now clay with an olive hill, a slate hill and a moon.