you need to set class name under Appearance > Menus section next to that menu and then under Theme options > Custom CSS & JS > Custom CSS section, you need to write css rules for that class name.
look at the attached picture, i want to change the top nav items from the mega menu to white, it would even be better if there was no hover on this but that is optional
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you need to set class name under Appearance > Menus section next to that menu and then under Theme options > Custom CSS & JS > Custom CSS section, you need to write css rules for that class name.
#Top_bar .menu > li > ul.mfn-megamenu a.mfn-megamenu-title { color: #FFF !important; }