Try this:

  body:has(dialog[open]) {
    overflow: hidden
  }

  body:has(dialog[open])::before {
    content: "";
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    backdrop-filter: blur(4px);
    background-color: rgb(0, 0, 0, 0.5);
  }