/// Convert an hexadecimal number to a decimal number
/// @access private
/// @param {String} $string - hexadecimal value
/// @return {Number} - decimal number
@function _hex-to-dec($string) {
  $hex: "0" "1" "2" "3" "4" "5" "6" "7" "8" "9" "a" "b" "c" "d" "e" "f";
  $string: to-lower-case($string);
  $length: str-length($string);

  $dec: 0;
  @for $i from 1 through $length {
    $factor: 1 + (15 * ($length - $i));
    $index: index($hex, str-slice($string, $i, $i));
    $dec: $dec + $factor * ($index - 1);
  }

  @return $dec;
}