After Jammy Jellyfish upgrade I am not able to customize the login screen - it comes with default MATE theme and has huge fonts probably because I have a HiDPI Monitor (ThinkPad T580). I have already tried to reinstall lightDM, slick-greeter and manually edit the /etc/lightdm config files with no effect.
Any help is much appreciated
You don't mention whether or not you've got the lightdm-settings utility, which isn't installed by default.
It utilizes slick-greeter, so I include that on the Terminal command-line for others...
sudo apt install lightdm-settings slick-greeter
Then look for Login Window in the main menu or in Control Center. It can configure numerous settings, including HiDPI support.
Thanks, I have used the settings utility at first. It showed me my previous login screen configuration which I event have tried to modify a bit in order to trigger changes. No success 
I finally got it fixed by simply removing and purging the arctica-greeter package.
There's another util that may tickle the config in a different way: LightDM GTK Greeter ...
sudo apt install lightdm-gtk-greeter-settings
When installed it appears as LightDM GTK+ Greeter settings in the Control Center.
