You can use gdm-tools to customize GDM:
To install:
sudo apt install libglib2.0-dev dconf-cli git
git clone --depth=1 https://github.com/realmazharhussain/gdm-tools.git
cd gdm-tools
./install.sh
Configure with:
set-gdm-theme list
I just logged into Gnome and tested Zorin Blue Dark and I do not have the same issue.
Have you Opened Zorin Appearance and clicked Reset to defaults?
Then re-select your theme.
You should be able to fully remove Big Sur theme by deleting its folder from /usr/share/themes or from ~/.themes or ~/.local/share/themes