How to change background of menu pop up on mobile view squarespace 7.1

Site URL: https://salamander-sealion-32lx.squarespace.com

Hi,

I'm trying to change the background of my menu pop up on mobile view. Currently it has no background colour and so the text is hard to read. Is there a way to change the background to just be white? Attached a photo of what it looks like currently. 

I've tried using both these codes but had no luck with either:

@media screen and (max-width:768px) {
div#overlayNav {
    background: white;
}
}

.header-menu-bg.theme-bg--primary {
    background: white;
}

Site url: https://salamander-sealion-32lx.squarespace.com

Password: DEMO

Thanks guys!

Add to Design > Custom CSS

/* Mobile popup menu */
.header-menu.header-menu--folder-list {
    background-color: white;
}