A light theme's accent as text on the always-dark chrome was 2.84:1 (Slate). `--chosen` is the
accent unless a scope names its own; `media-marks` (was media-focus) points it, with the focus
ring, at the media amber. A chosen track stays white with an amber tick (MenuCheck reads
`text-chosen`); a chosen settings tab or toggle is white with an amber underline (MEDIA_CHOSEN).
The theme contract test holds the token, the tick and the player components free of accent text.
ui/MenuCheck: a box filled with the accent for a set (a minus for a group
only partly in), a bare tick for one choice, in the row's leading slot.
Replaces seven hand-drawn marks: three box sizes, a tinted box, a cross
for "on" in the table filter, a trailing tick in the language menu.
peter