/// Provide an inset look to an element
/// @param {Number} $radius [0]
/// @param {Number} $top-opacity [0.3]
/// @param {Number} $bottom-opacity [0.25]
/// @param {Number} $bg-opacity [0.2]
@mixin o-emboss(

    $radius: 0,
    $top-opacity: 0.3,
    $bottom-opacity: 0.25,
    $bg-opacity: 0.2

  ) {

  background: rgba(0, 0, 0, 0.2);
  border-bottom: 1px solid rgba(#fff, $bottom-opacity);
  box-shadow: inset 0 3px 3px rgba(#000, $top-opacity);
  border-radius: $radius;
}