Mix the avatar palette from the active theme

Mix the avatar palette from the active theme

The seven grounds were fixed hexes, so a mark looked the same on every
Everforest variant and did not really belong to any of them. They are now
mixed from the active variant's own accents toward its panel colour, in
everforest.css, and the tone class just picks one. The disc is the
theme's moon or sun colours and the ink the hills shade toward is the
theme's darkest token.

Two shares of accent, because the panels are opposite: a dark theme's
panel is nearly black, so its grounds keep 80% of the accent to stay a
colour, while a light panel is nearly white and keeps 48% so the
landscape is pastel rather than a highlighter.

A custom property holding color-mix() reads back as text, so the bundle
resolves the palette through one hidden probe element and caches it per
theme; app.js already redraws the avatars when data-theme changes, so a
theme switch re-reads it. The fallback letter now takes --av-fg, which is
the theme's foreground, so it stays readable on a light theme's pastel
grounds.