I don't have much experience with linux but aren't the extensions* managed through Firefox? Logan used it in his "Logan's Linux Log" video -> open Firefox, enter http://extensions.gnome.org Not sure if this helps.
It's for older versions of Ubuntu (11.10 and 13.04) but it might be worth a try. The second answer (login page) seems to be the easiest solution that needs no tinkering (if it works at all).
edit: Also this one:
After installing the user theme extension, you should restart gnome-shell. Only then would the extension show up in the GNOME tweak tool.
To do that, press Alt + F2 and enter r in the dialog box. Now gnome-shell would restart and the extension will show up in the tweak tool.
See if the Metadata.json file is showing the right GNOME-Shell Version you're on.
Go into Bash and type
then type
Use Nano or Gedit whatever text editor you prefer (I use Gedit here) and see if the metadata.json file states the current GNOME version you're on if not make sure it says the version you're on then log out or reboot. it should fix it
Also forgot to mention if you don't already know how to check what GNOME version you're on, go into BASH and type