How to reset login screen to default on Ubuntu 17.10

Open Terminal and run

sudo update-alternatives --config gdm3.css

You should get something like this:

There are 2 choices for the alternative gdm3.css (providing /usr/share/gnome-shell/theme/gdm3.css).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/share/gnome-shell/theme/ubuntu.css        10        auto mode
* 1            /usr/share/gnome-shell/theme/gnome-shell.css   5         manual mode
  2            /usr/share/gnome-shell/theme/ubuntu.css        10        manual mode

Press <enter> to keep the current choice[*], or type selection number:

The asterisk (*) indicates the current choice, and it should be the gnome-shell.css one in your case.

Type the number (in the "Selection" column) associated to ubuntu.css and press enter.

(Source)