mobile menu Style 1 - dark transparent background

[Nzagamba]

I would like to darken the background of the website, when my mobile menu style 1, drops down.
The css rule of the background could be something like the bellow, but I couldn't figure out to what class or div I need to point this css to:

/* Transparent full-page overlay behind the menu */
.menu-overlay { /* what is the correct class???  */
  position: fixed;
  inset: 0;
  background: rgba(0, 0, 0, 0.5); 
  opacity: 0;
  transition: opacity 0.25s ease;
  z-index: 1000;