ColorScheme.toPureBlack() sets background, surface, surfaceDim and
surfaceContainerLowest to #000 and dims the other containers by scaling
their sRGB channels, so their hue tint carries over and grid tiles drawn
as containers over surface keep visible gaps. FloretExpressiveTheme takes
pureBlack to apply it to the dark scheme, dynamic or app-supplied.
core-prefs gains ThemeMode.BLACK (dark, pure black) and
ThemeMode.isPureBlack, stored under the same theme_mode key.
makiolaj
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