Custom overscroll colors for top and bottom sides of a page

overscrolls.sass

=overscrolls($topColor, $bottomColor, $darken: 10%)

	@if $topColor
		body::before
			content: ''
			position: fixed
			right: 0
			left: 0
			z-index: -1

			top: -50%
			bottom: 50%

			@if $darken
				background: $topColor linear-gradient($topColor 50%, darken($topColor, $darken))
			@else
				background: $topColor

	@if $bottomColor
		body::after
			content: ''
			position: fixed
			right: 0
			left: 0
			z-index: -1

			top: 50%
			bottom: -50%

			@if $darken
				background: $bottomColor linear-gradient(darken($bottomColor, $darken), $bottomColor 50%)
			@else
				background: $bottomColor