SASS mixin for :before and :after background images

lib.scss

@mixin before-after-background($height, $side-width, $before, $after) {
    height: $height;
  position: relative;
   z-index: 1;

  &:before, &:after {
     content: '';
     display: block;
      height: $height;
    position: absolute;
         top: 0px;
       width: $side-width;
     z-index: -1;
  }

  &:before {
    background-image: $before;
                left: -$side-width;
  }

  &:after {
    background-image: $after;
               right: -$side-width;
  }
}