I wonder if it's possible to change the appearance of the unlock screen as I'm not a big fan of this default greyish dialog box:

(Image reference: http://www.tuxgarage.com/2012/04/ubuntu-12-04-precise-pangolin-review.html)
I have literally no experience with applying any themes in Ubuntu and whenever I've tried to change a specific part of the interface I think I changed pretty much anything but the part I'd like to change. Therefore I'm also not sure if it's possible to just change this dialog box or if I've to apply a broader theme. Some explanation about the general structure would be very much appreciated.
Also some tips where to find fitting themes and how to apply them would come very handy - or just some links for tutorials explaining this task as I struggle to dig the net for some informational resource :)