This is a mock-up of the GNOME Tweak Tool style UI but with a better layout for changing GNOME Shell themes. The mock-up was done in Glade 3(user interface designer for GTK/GNOME) and some elements GIMPed.
Notes
Select a theme via the radiobutton.
The default theme option can be ticked at any time, which will put the shell theme back to the default one.
Install new theme allows you to install a new GNOME Shell theme which then adds it the top of the theme list.