Login screen theme and background lost

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http://askubuntu.com – I tried to change my LightDM background and theme in Ubuntu 12.04 LTS beta 2, changing /usr/share/glib-2.0/schemas/com.canonical.unity-greeter.gschema.xml file (I don't even know if it would work, but I couldn't find any other way), and I failed and restored original file. Also, durning the same session I installed gnome-shell-extensions package, but it didn't work either. (I didn't remove it) Now my login screen looks a bit coarse, without theme and background. (It looks like MS Windows 95 a bit - square buttons and text fields) My question is, how to repair login screen (or how to customi (HowTos)