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.
@luftman There is one setting for font-size that covers the whole site. The relative sizes are set by the theme, they'll scale up or down based on the selection from Small, Medium, Large, and Extra-Large.