Hi Nick,
Thanks for reaching out.
To center the navigation using the X theme would require customization. Regretfully, customization is outside the scope of our theme support. You may seek 3rd party developers to help you with your problems or you can avail One where we can answer questions outside of the features of our theme.
On the other hand, this layout can be done using Pro builder. In Pro, you can customize your header and footer based on the layout you wanted. I advised that you switch to Pro and here’s how you can do it. Please check it here.
The reason why your text is undersized on mobile because you are setting the text as em, I suggest that you set the text to px to maintain the sizes in the mobile.
Hope that helps.
Thank you.