Comment 3 for bug 1761014

Install Ubuntu 18.04 LTS with the minimum configuration specification.
If you execute the following, an error will occur.

$ sudo apt install vanilla-gnome-desktop
↓
Setting vanilla-gnome-default-settings (17.10.4) ...
update-alternatives: Error: alternative /usr/share/gnome-shell/theme/gnome-shell.css of gdm3.css is not registered. It does not set

In this state, if you execute the following, the error will be resolved.
$ sudo apt install ubuntu-gnome-desktop

---

So, from the beginning,
If you do the following, no error will occur.
$ sudo apt update
$ sudo apt install ubuntu-gnome-desktop
$ sudo apt install vanilla-gnome-desktop