This 4 minute video shows you how to enable font changes in Full Screen Mode in a text box. For bigger font changes (global aka site-wide, or your Navigation Heading, page headings, etc, then contact our Support Team with your specifics so they can let you know how much that custom change would cost (if anything).
BTW
One & the same thing:
Full Screen Composer / Full Screen Mode
…as well as:
Navigation Bar / Site Menu / Navigation Menu
Related FAQs: