To adjust the font for your lessons, click on the Go To menu and select "Display Settings".
Here, you can click the pencil icon to the right of your sections/tab names to adjust the font/size for your sections as a whole.
Adjust that and save the changes by clicking the checkmark icon in the upper-right corner.
**Please note - you won't see that font change when typing a lesson, only after applying or saving a lesson. **