just for confimation
as far i can read from tuto "Adding and Using Fonts"
Duplicate the TEMPLATE_DIR/gantry/theme.yaml file and place the copy in a safe place if you haven't done so already. As you will be modifying this file directly, and not through a custom override, you will likely lose these changes during an update.
not possible to overide file theme.yaml found in gantry/theme.yaml in gantry 5 ?