select2/docs/themes/learn2/scss/vendor/bourbon/addons/_rem.scss

@mixin rem($property, $size, $base: $em-base) {
  @if not unitless($base) {
    $base: strip-units($base);
  }

  $unitless_values: ();
  @each $num in $size {
    @if not unitless($num) {
      @if unit($num) == "em" {
        $num: $num * $base;
      }

      $num: strip-units($num);
    }

    $unitless_values: append($unitless_values, $num);
  }
  $size: $unitless_values;

  $pixel_values: ();
  $rem_values: ();
  @each $value in $pxval {
    $pixel_value: $value * 1px;
    $pixel_values: append($pixel_values, $pixel_value);

    $rem_value: ($value / $base) * 1rem;
    $rem_values: append($rem_values, $rem_value);
  }

  #{$property}: $pixel_values;
  #{$property}: $rem_values;
}