picocss/scss/utils/_box-shadow.scss

@function shadow-layer($elevation, $blur, $opacity, $color) {
  @return #{($elevation * 0.5)} #{$elevation} #{$blur} #{rgba($color, $opacity)};
}

@function shadow($color) {
  $box-shadow-elevation: 1rem;
  $box-shadow-blur-strengh: 6rem;
  $box-shadow-opacity: 0.06;

  @return shadow-layer(
      $box-shadow-elevation * 0.029,
      $box-shadow-blur-strengh * 0.029,
      $box-shadow-opacity * 0.283,
      $color
    ),
    shadow-layer(
      $box-shadow-elevation * 0.067,
      $box-shadow-blur-strengh * 0.067,
      $box-shadow-opacity * 0.4,
      $color
    ),
    shadow-layer(
      $box-shadow-elevation * 0.125,
      $box-shadow-blur-strengh * 0.125,
      $box-shadow-opacity * 0.5,
      $color
    ),
    shadow-layer(
      $box-shadow-elevation * 0.225,
      $box-shadow-blur-strengh * 0.225,
      $box-shadow-opacity * 0.6,
      $color
    ),
    shadow-layer(
      $box-shadow-elevation * 0.417,
      $box-shadow-blur-strengh * 0.417,
      $box-shadow-opacity * 0.717,
      $color
    ),
    shadow-layer($box-shadow-elevation, $box-shadow-blur-strengh, $box-shadow-opacity, $color),
    0 0 0 0.0625rem #{rgba($color, ($box-shadow-opacity * 0.25))};
}