::backdrop

button {
  font-size: 1.2rem;
  padding: 5px 15px;
}

dialog::backdrop {
  background-color: salmon;
}

<button id="showDialogBtn">Show a dialog</button>

<dialog id="favDialog">
  <form method="dialog">
    <p>The background shown outside of this dialog is a backdrop.</p>
    <button id="confirmBtn">Close the dialog</button>
  </form>
</dialog>

const showDialogBtn = document.getElementById("showDialogBtn");
const favDialog = document.getElementById("favDialog");

showDialogBtn.addEventListener("click", () => favDialog.showModal());

::backdrop {
  /* ... */
}

/* Backdrop is only displayed when dialog is opened with dialog.showModal() */
dialog::backdrop {
  background: rgb(255 0 0 / 25%);
}

<dialog>
  <button autofocus>Close</button>
  <p>This modal dialog has a beautiful backdrop!</p>
</dialog>
<button>Show the dialog</button>

::backdrop {
  background-image:
    radial-gradient(
      circle,
      white 0 5vw,
      transparent 5vw 20vw,
      white 20vw 22.5vw,
      #eeeeee 22.5vw
    ),
    conic-gradient(
      #272b66 0 50grad,
      #2d559f 50grad 100grad,
      #9ac147 100grad 150grad,
      #639b47 150grad 200grad,
      #e1e23b 200grad 250grad,
      #f7941e 250grad 300grad,
      #662a6c 300grad 350grad,
      #9a1d34 350grad 400grad,
      #43a1cd 100grad 150grad,
      #ba3e2e
    );
}

const dialog = document.querySelector("dialog");
const showButton = document.querySelector("dialog + button");
const closeButton = document.querySelector("dialog button");

// "Show the dialog" button opens the dialog modally
showButton.addEventListener("click", () => {
  dialog.showModal();
});

// "Close" button closes the dialog
closeButton.addEventListener("click", () => {
  dialog.close();
});