There's a much easier way to change the font by using Notepad++ itself. Go to:
"C:\Users[your username]\AppData\Roaming\Notepad++\stylers.xml" and open it. Then copy the string fontName="", press Ctrl + H and on the first field what you need to be found paste the copied string. On the second field (this tells you what you want to replace the copied string with), enter
fontName="Calibri" (or another font you like), click "replace everything" and DON'T FORGET TO TICK THE TICKBOX TO SEARCH ONLY FOR WHOLE WORDS BEFORE DOING ALL THAT.
After that, find the string
fontSize="" and replace it with
fontSize="11" (or any other size you prefer. Then save the file and restart NPP. This is how I did it, and here's the result:
Font: Calibri (my favourite font), font size: 11 (my favourite size for Calibri). Then archive the entire "C:\Users[your username]\AppData\Roaming\Notepad++\" to another drive (not on C:), so the next time you need to reinstall the OS, you'd be able to restore your settings on fonts.
This might sound oldschool to some people, but I'm an oldschool myself and this is the way I prefer to do it instead of relying on the program's options.