Just add the code below inside your page CSS:
body {
position: relative;
}
body:before{
content: '';
display: block;
position: absolute;
z-index: 2;
top: 0;
left: 0;
right: 0;
bottom: 0;
pointer-events: none;
background-image: url('https://library.elementor.com/resources/christmas-snow-effect/s1.png'), url('https://library.elementor.com/resources/christmas-snow-effect/s2.png'), url('https://library.elementor.com/resources/christmas-snow-effect/s3.png');
animation: snow 10s linear infinite;
}
@keyframes snow {
0% {background-position: 0px 0px, 0px 0px, 0px 0px;}
50% {background-position: 500px 500px, 100px 200px, -100px 150px;}
100% {background-position: 500px 1000px, 200px 400px, -100px 300px;}
}