darken method
Makes the color darker.
The higher the value(0.0 - 1.0), the darker the image becomes.
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();
}