When configuring the graphical greeter, there is a button \"Install new themes\". There could alsoi be a button like \"apt-get new themes\" which uses apt-get or synaptic to select , download and install more themes.
I'm afraid as this wasn't implemented for four years, it's not going to be done
Generally speaking, proper implementation isn't going to be limited with gdm,
it's much wider problem and it's not clear whether it's worth the trouble.