Are you experiencing Login Loop? Or is it that you want the original PW screen back?
Carmars instructions above for reinstalling GDM3 is the way toward getting your Login Screen back. Theming it back to Zorin will require another step than just installing.
To restore the Zorin Themed Splash screen, you must change the Plymouth theme.
In terminal enter
sudo update-alternatives --config default.plymouth
An editor will open in the terminal offering the avialble themes. To Select- type the NUMBER of the entry you want. For example, Zorin-Logo plymouth theme on mine is 0. So I would type a 0.
Once this is done, you must update the initramfs to see the change:
sudo update-initramfs -u
Are the other issues you were having repaired?
Also, I use Geany. It is very useful for editing .css files.
GDM3 login screen theme is located in /usr/share/gnome-shell/theme
I do not use Gnome or GM3, so you will need to look around in that directory for a zorin theme, if one is there. There should be a .css file in that directory that governs the GDM3 screen appearance.