_rem.scss

/**
 * Default rem baseline.
 * 
 * @type Number
 */
$guss-rem-baseline: 10px !default;

/**
 * Transform a value into rem.
 *
 * @param {Number} $value
 * @param {Number} $baseline ($guss-rem-baseline)
 *
 * @requires {variable} guss-rem-baseline
 *
 * @return {List | Number}
 */
@function rem($value, $baseline: $guss-rem-baseline) {
    @if $value == 0 {
        @return 0; // 0rem -> 0
    } 
    
    @if type-of($value) == list {
        $result: ();

        @each $e in $value {
            $result: append($result, rem($e, $baseline));
        }

        @return $result;
    } @else {
        @return if(type-of($value) == number and unit($value) == px, $value / $baseline * 1rem, $value);
    }
}

/**
 * Output `rem` units with `px` fallback.
 *
 * @link http://sassmeister.com/gist/7451284 Usage
 * 
 * @param {Map} $properties
 *
 * @requires {function} rem
 */
@mixin rem($properties) {
    @each $property, $value in $properties {
        @if (type-of($value) == number and $value != 0) {
            // Turn unitless values into pixels
            $value: if(unitless($value), $value * 1px, $value);
        }

        #{$property}: $value;
        #{$property}: rem($value);
    }
}