Add Verdant color theme with demo switcher ...

Palette variables become !default so themes can configure them.
kit-verdant.scss / demo-verdant.scss compile the green-shifted
palette (green-tinted neutrals, mint secondary, lime accent,
teal info) into kit-verdant.css / demo-verdant.css. Both demos
get a fixed theme switch in the corner that swaps the theme
stylesheets and persists the choice. The default Tokyo Night
theme is unchanged.