/etc/lightdm/mythbuntu-lightdm-gtk-greeter.conf is in mythbuntu-lightdm-theme 1.01.
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 | [greeter]
background=/usr/share/images/mythbuntu/bg_2560x1600.jpg
theme-name=Dust
font-name=Ubuntu 11
xft-antialias=true
xft-dpi=96
xft-hintstyle=slight
xft-rgba=rgb
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