I was just goofing around in a vm to test what would happen if I purgen unity:

sudo apt-get remove --purge unity*

I found that I get an startup error, but when I proceed, I got this awesome login screen:

It has an sexy effect and is just the gnome style my whole pc has got.

So can any one please tell me how I change the default login screen program without having to bother clicking "fix this error" on every boot?

Thanks in advance.

[EDIT]:

I have been able to change it(my own answer), but now I see the old purple background for a second when I logged in, and that background is only used in the old greeter screen.
How do I change it(or where are those backgrounds located)?