Yes, you need to add custom CSS to file style.css( child-theme):
//Font size and color
.mobile-navigation ul>li.menu-item>a, .mobile-navigation-categories ul>li.menu-item>a{
font-size: your-size !important;
color: #your-color !important;
}
Activate color:
.mobile-navigation ul.menu li.current-menu-item>a, .mobile-navigation ul.menu li.current-menu-ancestor>a, .mobile-navigation ul.menu li.current-menu-parent>a, .mobile-navigation-categories ul.menu li.current-menu-item>a, .mobile-navigation-categories ul.menu li.current-menu-ancestor>a, .mobile-navigation-categories ul.menu li.current-menu-parent>a{
color: #your-color !important;
}