Hi,

Try to use this selector to control the color of your mobile sub-menus background

/* bg color of sub-menu */

body .mobile-nav .sub-menu-dropdown ul {
  background-color: gray;
}

body .mobile-nav .sub-menu-dropdown ul:before {
	color: gray;
}

/* color of sub-sub-menu */

body .mobile-nav .sub-menu-dropdown li ul {
	background-color: black;
}

body .mobile-nav .sub-menu-dropdown li ul:before {
	color: black;
}

Regards