Lightens and darkens a material Color. Taken from a solution from StackOverflow.

color_degree_manipulator.dart

import 'package:flutter/material.dart';

extension ColorDegree on Color {
  Color darken([int percent = 10]) {
    assert(1 <= percent && percent <= 100);
    var f = 1 - percent / 100;

    return Color.fromARGB(
        this.alpha, (this.red * f).round(), (this.green * f).round(), (this.blue * f).round());
  }

  Color lighten([int percent = 10]) {
    assert(1 <= percent && percent <= 100);
    var p = percent / 100;
    this.withRed(this.red + ((255 - this.red) * p).round());
    this.withGreen(this.green + ((255 - this.green) * p).round());
    this.withBlue(this.blue + ((255 - this.blue) * p).round());
    return Color.fromARGB(this.alpha, this.red + ((255 - this.red) * p).round(),
        this.green + ((255 - this.green) * p).round(), this.blue + ((255 - this.blue) * p).round());
  }
}