You need to edit the /etc/default/grub file and then run update-grub
The /etc/default/grub.ufc-dist file is used by ufc for preserving user changes to the config file.
https://packages.debian.org/stretch/ucf
The /etc/default/grub.ufc-dist file is used by ufc for preserving user changes to the config file.
https://packages.debian.org/stretch/ucf