// darkens the @color by adding
// black on top with @alpha opacity (0 - 1)
.darken(@color,@alpha) {
  background: linear-gradient(to top, rgba(0, 0, 0, @alpha), rgba(0, 0, 0, @alpha)) @color;
}

// Lighten the @color by adding
// white on top with @alpha opacity (0 - 1)
.lighten(@color,@alpha) {
  background: linear-gradient(to top, rgba(255, 255, 255, @alpha), rgba(255, 255, 255, @alpha))
    @color;
}

// changes the brightness of given element
// darken 0- black to 1 - no change
// lighten 1 - no change to no limit
.brightness(@amount) {
  filter: brightness(@amount);
}