Is it possible to add a custom theme to the GDM Greeter login screen on RHEL6?

Issue

• Does the simple-greeter in RHEL6 has theming option?
• The gdm simple-greeter window is put to center screen by default. Can the login window be put to top left of the window?
• I had tried to add -x 10 -y 10 to Exec=/usr/libexec/gdm-simple-greeter in gdm-simple-greeter.desktop but that does not work.
• Some other option is GdmGreeterThemes but I'm not sure if this is working in RHEL6.