Flatiron - Change text of "menu" in navigation

I am using the Flatiron template and in mobile view, I would like the navigation to say "+" instead of "menu"

I do not want the navigation to change in desktop view.

my website is www.isissakomadajohn.com

Thanks!

Add to Home > Design > Custom CSS

@media screen and (max-width:640px) {
#mobile-navigation #mobile-navigation-label:after, #mobile-navigation.sqs-mobile-nav-open #mobile-navigation-label:after {
    content: "+";
    font-size: 50px;
}
}

thanks!!