darken method
This will return the darker color of current amount must be between 0 & 1
Implementation
Color darken([double amount = 0.1]) {
assert(amount >= 0 && amount <= 1);
final hsl = HSLColor.fromColor(this);
var lightness = (hsl.lightness - amount).clamp(0.0, 1.0);
final hslLight = hsl.withLightness(lightness);
return hslLight.toColor();
}