How can I change default font Notes - Sierra, High Sierra
Yes. You have to close out of the Notes application and go to the Notes app in the Applications folder. Right-click on the Notes app and select 'Show Contents" From inside the Contents folder open the Resources folder and inside this folder is a folder named en.lproj. This is for the English language. Other similar folders exist for other languages. Inside this folder is a file named DefaultFonts.plist. Edit this file. The file is locked so editing it can be tricky.This link has a full description of how to do it all.  The link will fill in missing details.