/etc/lightdm/lightdm-gtk-greeter-ubuntu.conf is in lightdm-gtk-greeter 1.1.5-0ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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# background = Background file to use, either an image path or a color (e.g. #772953)
# theme-name = GTK+ theme to use
# font-name = Font to use
# xft-antialias = Whether to antialias Xft fonts (true or false)
# xft-dpi = Resolution for Xft in dots per inch (e.g. 96)
# xft-hintstyle = What degree of hinting to use (hintnone, hintslight, hintmedium, or hintfull)
# xft-rgba = Type of subpixel antialiasing (none, rgb, bgr, vrgb or vbgr)
# show-language-selector (true or false)
#
[greeter]
background=/usr/share/backgrounds/warty-final-ubuntu.png
theme-name=Ambiance
font-name=Ubuntu 11
xft-antialias=true
xft-dpi=96
xft-hintstyle=slight
xft-rgba=rgb
show-language-selector=false
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