The default popover is glass over the app's own chrome, which is right for
something the user opened and wrong for something the app said. The accent
variant fills the same box — same arrow, same placement engine — with a solid
brand colour so an unprompted surface reads as the app speaking.
Filling it with `primary` directly doesn't work across themes: a pale accent is
a perfectly valid primary (imported VS Code themes love a pastel), and the
honest `primaryForeground` for one is near-black, so the loud surface comes out
a pastel card whispering. `--dt-primary-solid` deepens the hue until a light
foreground clears AA — a no-op on an accent that is already deep, and darkening
only, so the hue survives.
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