How to select lightdm greeter theme or appearance?

Question

I'm testing Ubuntu 20.04 LTS with many desktop environments installed.

Currently I have the following:

$ dpkg -l | grep -i greet
ii  arctica-greeter                                             0.99.1.4-1                                      amd64        LightDM Arctica Greeter
ii  arctica-greeter-guest-session                               0.99.1.4-1                                      all          LightDM Arctica Greeter - Guest Session Support
ii  kylin-greeter                                               19.04.2                                         amd64        Ubuntu Kylin Greeter
ii  lightdm-autologin-greeter                                   1.0-3                                           all          Autologin greeter for LightDM
ii  lightdm-gtk-greeter                                         2.0.6-0ubuntu1                                  amd64        simple display manager (GTK+ greeter)
ii  lightdm-gtk-greeter-settings                                1.2.2-3                                         all          settings editor for the LightDM GTK+ Greeter
ii  slick-greeter                                               1.3.2+dfsg.1-1                                  amd64        Slick-looking LightDM greeter
ii  ukui-greeter                                                1.2.5-1                                         amd64        Lightdm greeter for UKUI
ii  unity-greeter                                               18.04.0+20.04.20200312-0ubuntu2                 amd64        Unity Greeter
ii  unity-greeter-badges                                        0.4-0ubuntu3                                    all          Additional badges for Unity Greeter

$ ls /usr/share/xgreeters/ -al
total 104
drwxr-xr-x    2 root root  4096 апр 30 22:05 .
drwxr-xr-x 2696 root root 69632 мая  1 00:25 ..
-rw-r--r--    1 root root  3057 дек  3 00:29 arctica-greeter.desktop
-rw-r--r--    1 root root   131 фев 27 21:54 kylin-greeter.desktop
-rw-r--r--    1 root root   206 авг 17  2018 lightdm-autologin-greeter.desktop
lrwxrwxrwx    1 root root    33 апр 23 17:11 lightdm-greeter.desktop -> /etc/alternatives/lightdm-greeter
-rw-r--r--    1 root root   187 сен 18  2018 lightdm-gtk-greeter.desktop
-rw-r--r--    1 root root   131 мар  3 23:39 slick-greeter.desktop
-rw-r--r--    1 root root   127 апр  7 09:43 ukui-greeter.desktop
-rw-r--r--    1 root root   131 апр  2 12:58 unity-greeter.desktop

$ update-alternatives --config lightdm-greeter 
There is 1 choice for the alternative lightdm-greeter (providing /usr/share/xgreeters/lightdm-greeter.desktop).

  Selection    Path                                              Priority   Status
------------------------------------------------------------
  0            /usr/share/xgreeters/lightdm-gtk-greeter.desktop   60        auto mode
* 1            /usr/share/xgreeters/lightdm-gtk-greeter.desktop   60        manual mode

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

Currently I see that the system uses Arctica styling which does not allow to scroll the list of available graphical sessions.
How can I switch it to different one?

I tried to use lightdm-settings, lightdm-gtk-greeter-settings-pkexec and sudo dpkg-reconfigure lightdm, sudo dpkg-reconfigure lightdm-gtk-greeter, sudo dpkg-reconfigure lightdm-gtk-greeter-settings, but no luck.

Answer 1

This is a case when configuration is possible by package removal. I can't find other solution.
So we need to remove almost all lightdm greeter packages:

sudo apt-get purge arctica-greeter arctica-greeter-guest-session
sudo apt-get purge kylin-greeter ukui-greeter
sudo apt-get autoremove --purge

and (re)install the most usable one:

sudo apt-get install --reinstall ubuntu-mate-lightdm-theme

and enjoy scrollable list of all available desktop sessions:

Answer 2

I was able to switch greeter (Ubuntu 20.04) by creating the /etc/lightdm/lightdm.conf file and adding this line to select the lightdm-gtk-greeter for example:

[SeatDefaults]
greeter-session=lightdm-gtk-greeter

source: How to change the LightDM theme/greeter?