Pixel to Rem mixin. From: http://miekd.com/articles/sizing-type-leading-and-vertical-dimensions-with-rem-units/

rem_mixin.scss

$base_font_size: 16px;

@mixin rem($property, $px_values) {
    $baseline_rem: ($base_font_size / 1rem);
    #{$property}: $px_values;

    @if type-of($px_values) == 'number' {
        #{$property}: $px_values / $baseline_rem;
    } 
    @else {
        $rem_values: ();

        @each $value in $px_values {
            @if $value == 0 {
                $rem_values: append($rem_values, $value);
            } @else {
                $rem_values: append($rem_values, ($value / $baseline_rem) );
            }

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