The default font for Notes is set in the Styles area of the Extras drawer when a new Note is created. If your default font style was Simple 12 point plain when you created the Note, you can select the type and change it using Styles, but it will not be the default font for that Note. If you set Styles to Fancy 18 point Bold, then create a new Note, the default font will be Fancy 18 point Bold.
The Names, Dates and ToDo font are preset to specific defaults. The default font in these areas cannot be changed.