Hello,
Try to add that code to child theme style.css
.mobile-menu-wrapper .menu > li > a {
color: #fff !important;
}
Clear the plugin cache after that and check.
Also, could you, please, inspect the element using Chrome inspector tool and check if you have the following styles for the mobile menu links http://prntscr.com/o9eq6e Would be great if you could provide a screenshot or video with styles that you have for that element while inspecting the menu.
Regards