%overlay {
    bottom: 0;
    left: 0;
    position: absolute;
    right: 0;
    top: 0;
  }

@mixin overlay($offset: 0) {
    @extend %overlay;
  
    @if $offset != 0 {
      bottom: $offset;
      left: $offset;
      right: $offset;
      top: $offset;
    }
  }