How to use the mixin

_form.scss

// usage of the mixin

select:before {
			content: '';
			background: $lightGrey src (resource_url('img/dropdown.png')) center center no-repeat;
			background-size: 75%;
			height: $baseUnit*3;
			width: $baseUnit*3;
			z-index: 3;
			position: absolute;
			top: 0;
			right: 0;
		}