How to use if-resolution SASS mixin

if-resolution-example.scss

footer {
  // only include 1x size image for 'non-retina' screens
  @include if-max-resolution(1.49) {
    background-image: url('img/[email protected]');
  }
  // only include 2x size image for 'retina' screens
  @include if-min-resolution(1.5) {
    background-image: url('img/[email protected]');
    background-size: 10.5625rem 1.5625rem;
  }
}