:root { --grain-opacity: .06; } /* tweak strength */
html, body { height:100%; background:#0e4d2b; } /* your green */
body::before{
content:"";
position:fixed; inset:0;
pointer-events:none;
z-index:999999; /* above everything, but not clickable */
opacity:var(--grain-opacity);
mix-blend-mode:multiply; /* try overlay/soft-light too */
background-image: url("data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAgAAAAICAYAAADED76LAAAAL0lEQVQYV2NkYGD4z0AbYMQgRRCgGJgYGBhGJgZ/ABQxgQGg2kCw0EwDgYQzCwBQAQAA8n0V2b0o3jQAAAABJRU5ErkJggg==");
background-repeat:repeat;
image-rendering: pixelated; /* keeps noise crisp */
}