$list: (black);
$list: join($list, white, comma);

div {
  padding: join(10px 20px, 30px 40px);
  margin: join((0, 10px), (10px, 10px), space);
  background: linear-gradient($list);
}

$list: ();
$list: join($list, (red, blue), comma);

p {
  background: linear-gradient($list);
}