genericons on main menu

I am currently using one menu (Main) containing links to social media. Setting up custom links for social media in Appearance/Menus for social menu automatically results in display of icons for links. Setting up custom links for social media for main menu results in display of text for links. How can I change to display icons on main menu? I see in custom.css file the info used for social media, but am unsure where/how to apply to main menu. Thanks!