Снег на фоне

body {
    background: #4e5257 !important;
    }

body:before {
    content: "";
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background-image:   url("https://i.imgur.com/TpOI4DC.png"),
                        url("https://i.imgur.com/92lgmBN.png"),
                        url("https://i.imgur.com/ACiIkIG.png") !important;
    background-size: 250px, 200px, 150px;
    animation: snow 16s linear infinite;
}

@keyframes snow {
    0% {
        background-position:    0px 0px,
                                0px 0px,
                                0px 0px;
    }
    100% {
        background-position:    500px 1000px,
                                400px 400px,
                                300px 300px;
    }
}