Recently I installed Ubuntu 12.04 on a virtual machine. I didn't want to have Unity running so I searched for a copy of Gnome 3. It had been a while since I had done my first install of gnome on my actual boot of linux, so I installed the first result I saw. That happened to be gnome-session-fallback or whatever. I tried to uninstall it and reinstall gnome 3 but now, whenever I log in, I'm still in fallback mode and don't have any options for shell themes in gnome tweak tool. What do I do?