You can take this selector body .navigation-style-underline .menu-mega-dropdown .color-scheme-dark .sub-sub-menu li a and use it in the Advance typography as the custom selector.
Alternatively, you can insert this custom code to the Theme Settings > Custom CSS > Global:
body .navigation-style-underline .menu-mega-dropdown .color-scheme-dark .sub-sub-menu li a {
color: #777;
}