Hello tgueth, I hope you are well. You can change Font/Size in Nav Bar easily. But there are basic options including a global font-size scale (affects all text proportionately), as well as color selections. Advanced customization is not available in the current edition of GoCentral. Follow this guide ;
The font sizes are based on a global setting in the theme options. They'll scale based on the selection of Small, Medium, Large, and Extra Large.