withBrightness method
def with its brightness draw fixed to the intent's, when def can
draw it (engine v5 families with their own palettes).
Implementation
FamilyDefinition withBrightness(FamilyDefinition def) {
final b = brightnessFor(def);
if (b == null) return def;
return def.revise(
brightnessWeights: b == ThemeBrightness.dark
? const [1, 0]
: const [0, 1],
);
}