The login screen appears in a much older style:
How can I change it back to normal?
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You have set your default display manager to
xdm in place of the default more modern
You should be able to change back to your preferred display manager by reconfiguring the package using
sudo dpkg-reconfigure xdm
and then navigating the menu with your arrow and TAB keys and confirming your selection with the ENTER key or SPACEBAR.
The choice will take effect next time you reboot - or you can stop then re-start the display manager manually using
systemctl (note that this will terminate any running desktop session as well):
sudo systemctl isolate multi-user.target
sudo systemctl isolate graphical.target