edx-platform/static/vendor/bourbon/css3/_linear-gradient.scss

@mixin linear-gradient($pos, $g1, $g2: null,
                       $g3: null, $g4: null,
                       $g5: null, $g6: null,
                       $g7: null, $g8: null,
                       $g9: null, $g10: null,
                       $fallback: null) {
  // Detect what type of value exists in $pos
  $pos-type: type-of(nth($pos, 1));
  $pos-spec: null;
  $pos-degree: null;

  // If $pos is missing from mixin, reassign vars and add default position
  @if ($pos-type == color) or (nth($pos, 1) == "transparent")  {
    $g10: $g9; $g9: $g8; $g8: $g7; $g7: $g6; $g6: $g5;
    $g5: $g4; $g4: $g3; $g3: $g2; $g2: $g1; $g1: $pos;
    $pos: null;
  }

  @if $pos {
    $positions: _linear-positions-parser($pos);
    $pos-degree: nth($positions, 1);
    $pos-spec:   nth($positions, 2);
  }

  $full: $g1, $g2, $g3, $g4, $g5, $g6, $g7, $g8, $g9, $g10;

  // Set $g1 as the default fallback color
  $fallback-color: nth($g1, 1);

  // If $fallback is a color use that color as the fallback color
  @if (type-of($fallback) == color) or ($fallback == "transparent") {
    $fallback-color: $fallback;
  }

  background-color: $fallback-color;
  background-image: -webkit-linear-gradient($pos-degree $full); // Safari 5.1+, Chrome
  background-image: unquote("linear-gradient(#{$pos-spec}#{$full})");
}