darken method
Returns a darker version of this color by amount (between 0.0 and 1.0).
Implementation
Color darken([double amount = 0.1]) {
assert(amount >= 0 && amount <= 1);
final hsl = HSLColor.fromColor(this);
final hslDark = hsl.withLightness((hsl.lightness - amount).clamp(0.0, 1.0));
return hslDark.toColor();
}