darken method
Implementation
TinyColor darken([int amount = 10]) {
final p = RangeError.checkValueInInterval(amount, 0, 100, 'amount') / 100;
final hsl = toHSLColor();
final lightness = clamp01(hsl.lightness - p);
_color = hsl.withLightness(lightness).toColor();
return this;
}