Hey Corinne,
Regretfully, code is needed in X header. The icon options in the menu you see are for the Navigation elements which you can use also in the header if you use Pro which I’d recommend that you upgrade to so that you stop using code for this case.
I see that you have this HTML in your menu <i class="x-icon-book-s" data-x-icon-s=""></i>. The icon specific class is no longer applicable. You need to use the general x-icon class instead. For the data attribute, insert &#x followed by the icon’s unicode which you can find in the icon page in FontAwesome like this https://fontawesome.com/icons/book?style=solid
What I’ve showed you should work provided you follow it correctly. Please see the secure note.
Hope that helps.