Fade Main Background when Navigation Folder is Active

Site URL: https://pepper-bagpipe-m9kf.squarespace.com/

Hello - I would like to add custom CSS that will fade the site's background (by placing a transparent overlay over it) when a folder in the navigation is active.  I don't like the options to style the text in the folder or the color of the folder itself in order to see it against my site's background.  Instead, I want it to look like the site fades out slightly when a folder is active.  I found the code below to do this when the mobile bar is active, but can't figure out how to do it otherwise.  Thank you!

body.is-mobile-overlay-active: after
{
content: ' ';
width: 100%;
height: 100%;
position: absolute;
top: 0;
left: 0;
background: rgba (0,0,0,.5);
z-index:500;
}
 

password: MRphotos