Lead Developer
The search in the navigation bar mimics whatever settings you have set for your menu items, so you’ll need to match that CSS to your new icon.
To do that, you’ll probably need to wrap your icon in a link: <a href="#" class="custom-search"><i class="fa fa-search fa-flip-horizontal"><!– icon –></i></a>
Documentation: http://docs.generatepress.com/
Adding CSS: http://docs.generatepress.com/article/adding-css/
Ongoing Development: https://generatepress.com/ongoing-development