Here's a picture of my login screen. (Please excuse the Moiré bands;
it's an actual photograph.)
https://drive.google.com/open?id=0B96WA_V8sR9bRzFKc2RfS1NmTlU
It doesn't look anything like the images in
System Settings->Startup and Shutdown->Login Screen
Note, in particular, the X-Windows style X shaped cursor.
It seems that part of a theme is missing; but I have no idea what it
could be.
Thanks - jon