Login screen appears in a much older style

You have set your default display manager to xdm in place of the default more modern lightdm or gdm

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

then

sudo systemctl isolate graphical.target