Firefox Developer Tools report this CSS being applied to top-left logo, and it seems to work. Not sure if there is anything else that is missing.
.logo-small {
filter: drop-shadow(1px 1px 1px #ddd) drop-shadow(-1px 1px 1px #ddd) drop-shadow(1px -1px 1px #ddd) drop-shadow(-1px -1px 1px #ddd);
}
.logo-big {
filter: drop-shadow(1px 1px 1px #ddd) drop-shadow(-1px 1px 1px #ddd) drop-shadow(1px -1px 1px #ddd) drop-shadow(-1px -1px 1px #ddd);
}