Ubuntu – install gnome-shell on ubuntu 16.04

It seems you've logged into an (default) Ubuntu session somehow instead of a (vanilla) GNOME session.

To do that when you boot your system and get to the GDM login screen you should find a cogwheel (⚙️) next to the sign in button. If you click on the cogwheel you should find an GNOME (and GNOME on Xorg) option. Select it and log in.

If you want to adapt my answer from the post you linked, you need add the following line to the created gnome-shell.css file

@import url("/usr/share/gnome-shell/theme/gnome-shell.css");

instead of the

@import url("/usr/share/gnome-shell/theme/ubuntu.css");

line.

As suggested by the author of question @Crantisz, to make the topbar always opaque, add the following to the created gnome-shell.css file.

@import url("/usr/share/gnome-shell/theme/gnome-shell.css");

#panel {
  background-color: rgba(0, 0, 0, 1);
}

On a whim, because I had seen some updates to the nvidia drivers, I tried to switch to nouveau. I have no idea if I did so correctly, but it solved the problem. This is what I ran:

sudo apt-get purge nvidia-\*
sudo apt-get autoclean
sudo apt-get autoremove
sudo reboot

At this point, everything seems to be working again, and looking at the "Additional Driver" tab of "Software & Updates", I see that this computer is now using Nouveau instead of NVIDIA.