customizing login screen theme in 20.04

What is the output of:

dpkg -l | egrep 'lightdm|gdm|kdm|slim|xdm'

Thanks

$ dpkg -l | egrep 'lightdm|gdm|kdm|slim|xdm'
ii gdm3 3.34.1-1ubuntu1 amd64 GNOME Display Manager
ii gir1.2-gdm-1.0:amd64 3.34.1-1ubuntu1 amd64 GObject introspection data for the GNOME Display Manager
ii libgdm1 3.34.1-1ubuntu1 amd64 GNOME Display Manager (shared library)
ii libxdmcp-dev:amd64 1:1.1.3-0ubuntu1 amd64 X11 authorisation library (development headers)
ii libxdmcp6:amd64 1:1.1.3-0ubuntu1 amd64 X11 Display Manager Control Protocol library
ii libxdmcp6:i386 1:1.1.3-0ubuntu1 i386 X11 Display Manager Control Protocol library

Seems you are using gdm2 so any customisation for gdm3 will work with your OS

Gnome-Look has some:
https://www.gnome-look.org/browse/cat/131/

It's not something I bother with myself as it's on the screen so very little time

its not gdm2 its gdm3 the latest version

Read your own output....

gdm3 3.34.1-1ubuntu1 amd64 GNOME Display Manager

it says gdm3 3.34.1

Ahh sorry. I mistyped. Yes it's gdm3 and I wrote gdm2. My mistake. Apologies

so how to customize gdm3 login screen in ubuntu 20.04?

Use the Gnome-Look page I linked

i tried that before and it doesn't work
i've try to look for the css file but it doesn't exists
i even try every solution written in the internet for older version of ubuntu 19.10 doesn't work

Does the gnome-tweak app help?

yes help with customizing everything after the login screen

but it doesn't help with the login screen

looking for a way to customize my login screeen in ubuntu 20.04
i reached to this file as login screen theme "/etc/alternatives/gdm3-theme.gresource"
it used to be css file where editing is easier but with the gresource file i cant get it to work.
i try to extract and regenrate the gresource file again but it doesn't work
i used this script to extract the data :

#!/bin/sh
gst=gnome-shell-theme.gresource
workdir=./shell-theme

for r in `gresource list $gst`; do
    r=${r#\/org\/gnome\/shell/}
    if [ ! -d $workdir/${r%/*} ]; then
      mkdir -p $workdir/${r%/*}
    fi
done

for r in `gresource list $gst`; do
        gresource extract $gst $r >$workdir/${r#\/org\/gnome\/shell/}
done

then i generate gnome-shell-theme.gresource.xml
then i run this command

$ glib-compile-resources gnome-shell-theme.gresource.xml

but nothing changed

Thanks bernard stafford, that solved my question.