@mixin box-shadow($shadow...) { @if $enable-shadows { $result: (); @each $value in $shadow { @if $value != null { $result: append($result, $value, "comma"); } @if $value == none and length($shadow) > 1 { @warn "The keyword 'none' must be used as a single argument."; } } @if (length($result) > 0) { box-shadow: $result; } } }