Customizing GDM3 Themes in Ubuntu 20.04 Focal Fossa

After upgrading to the latest LTS Ubuntu 20.04, I decided to stop using the old standard lightdm-gtk-greeter and give in to using Gnome’s gdm3. But I don’t like Ubuntu’s default styles — in fact, I use a vanilla gnome-session entirely to avoid the purple flash you get on ubuntu-session. After some research, I found you