Styling issue with custom menus and sub menus

You can try using the .current-menu-item li a”first-child selector. Here is an example I’ve used in the past to highlight the current menu item only:

#menu .current-menu-item {
background:white; 
}
#menu .current-menu-item li a:first-child { 
color:white!important;
}
#menu .current-menu-item a { 
color:black !important ;
}

You will need to change the #menu to your id or class.