Set default font is only for all html editors in the application like like incident description, resolution area, reply templates, resolution templates, all notifications and etc wherever html editors are present and NOT to set a default font throughout the application.
Please take a backup or a SQL backup or a server snapshot (whichever is faster) before executing the update query.
To change the font in all html editors to the desired font, please connect to the database and execute the below query update globalconfig set paramvalue='Calibri' where category='rta' and parameter='font-family';
update globalconfig set paramvalue=12 where category='rta' and parameter='font-size';
In the above query, the font is mentioned as Calibri, kindly change to the desired font with the right spelling and execute the query.
Also, only the size that is listed in the UI can be set (like 10,12,14 etc). For instance, 11 is not there, if you try to set the same, it will take 12 as the default value.
Navigate to Community tab-> Support tool-> Type 'GLOBALCONFIG' and hit Drop Cache for the changes to be effective.
After dropping the cache, refresh the page to check for the change of font.