/usr/share/lightdm/lightdm-gtk-greeter.conf.d/30_xubuntu.conf is in xubuntu-default-settings 18.04.6.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 | # Xubuntu specific defaults
# Settings stack on top of 01_ubuntu.conf
#
[greeter]
background=/usr/share/plymouth/themes/xubuntu-logo/wallpaper.png
theme-name=Greybird
icon-theme-name=elementary-xfce-dark
font-name=Noto Sans 9
keyboard=onboard
screensaver-timeout=60
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