Jack Harner's Box Shadow SCSS Mixin

_cardShadow.scss

/// Computes a top-shadow for a card effect.
/// @param {Number} $depth - depth level
/// @return {List}
@function top-shadow($depth) {
    $primary-offset: nth(1 3 10 14 19, $depth) * 1px;
    $blur: nth(2 3 10 14 19, $depth) * 4px;
    $color: rgba(black, nth(0.12 0.16 0.19 0.25 0.3, $depth));

    @return 0 $primary-offset $blur $color;
}

/// Computes a bottom-shadow for a card effect.
/// @param {Number} $depth - depth level
/// @return {List}
@function bottom-shadow($depth) {
    $primary-offset: nth(1.5 3 6 10 15, $depth) * 1px;
    $blur: nth(1 3 3 5 6, $depth) * 4px;
    $color: rgba(black, nth(0.24 0.23 0.23 0.22 0.22, $depth));

    @return 0 $primary-offset $blur $color;
}

@mixin card($depth) {
    @if $depth < 1 {
        box-shadow: inset bottom-shadow(abs($depth)), inset top-shadow(abs($depth));
    }
    @else if $depth>5 {
        @warn "Invalid $depth `#{$depth}` for mixin `card`.";
    }
    @else {
        box-shadow: bottom-shadow($depth), top-shadow($depth);
    }
}