The problem is that when you save the changes that you made directly in the style.css the Font Awesome icons characters get lost (probably because of the editor encoding settings).

You can solve that if you put your custom CSS in the wp-admin -> Customizer -> Additional CSS or if you create a child theme. Once you transfer all your custom CSS in one of those two options than you can overwrite the original style.css with the fresh copy from your ProteusThemes dashboard.

If you have issues with Font Awesome version 5.x in our themes, please read this help article.