/usr/share/lightdm/lightdm-gtk-greeter.conf.d/01_debian.conf is in lightdm-gtk-greeter 2.0.2-1.
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 | # Debian specific defaults
#
[greeter]
background=/usr/share/images/desktop-base/login-background.svg
theme-name=Adwaita
xft-antialias=true
xft-hintstyle=hintfull
xft-rgba=rgb
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