I've followed the steps posted here with intentions of installing a custom theme downloaded from the previous site.

When I get to the step:

You’ll now need to log out and back in for the extension to be ‘picked
up’ by the system, then enable the extension from the ‘Shell
Extensions’ tab in the ‘Advanced Settings/GNOME Tweak Tool’ app.

I open GNOME Tweak Tool and navigate to the Shell Extensions tab but get the following:

What am I doing wrong? I cannot see the necessary options to enable shell-extension themes.