This code will target the opacity on Divi's navigation bar links. Place this code inside your style.css file in your child theme's directory.

Control Opacity on Divi's Navigation Bar

#et-info-email:hover, 
#et-secondary-menu > ul > li > a:hover, 
#top-menu-nav > ul > li > a:hover, 
.et-social-icons a:hover {
    opacity: 1;
    -webkit-transition: all .4s ease-in-out;
    -moz-transition: all .4s ease-in-out;
    transition: all .4s ease-in-out;
}