The PR showed two sheets for one theme, which cannot show that the shipped
reds and greens stay legible on every palette. Each shipped theme and scheme
now gets its own sheet: both families, three glyphs, at 16 and 24 px, on the
window and button surfaces the contrast is measured against, with the
disabled tone beside each family.
The generator lives in its own test file so the preview work does not have to
move the branches above it.
