Simple mixin to return the property with REM units along with PX fallback.

mixins.scss

$font-size: 16;

@function strip-unit($num) {
  @return $num / ($num * 0 + 1);
}

@mixin rem($property, $values...) {
  $max: length($values);
  $pxValues: '';
  $remValues: '';

  @for $i from 1 through $max {
    $value: strip-unit(nth($values, $i));
    $pxValues: #{$pxValues + $value}px;

    @if $i < $max {
      $pxValues: #{$pxValues + " "};
    }
  } 

  @for $i from 1 through $max {
    $value: strip-unit(nth($values, $i));
    $remValues: #{$remValues + ($value/$font-size)}rem;

    @if $i < $max {
      $remValues: #{$remValues + " "};
    }
  } 
  
  #{$property}: $pxValues; 
  #{$property}: $remValues; 
}