color-shifts.less

.background-color-shift (@color, @amount: 10%) when (lightness(@color) > 50%) {
	background-color: darken(@color, @amount);
}
.background-color-shift (@color, @amount: 10%) when (lightness(@color) <= 50%) {
	background-color: lighten(@color, @amount);
}
.text-color-shift (@color, @amount: 10%) when (lightness(@color) > 50%) {
	color: darken(@color, @amount);
}
.text-color-shift (@color, @amount: 10%) when (lightness(@color) <= 50%) {
	color: lighten(@color, @amount);
}

replace <= with =<.

In less the comparisons match on this: /^(?:>=|=<|[<=>])/

Thanks, @fat. I forgot LESS comparisons worked that way.

No worries :)