Install User theme from Gnome extensions website. Which is accomplished by clicking the OFF button (top left). It will ask for your permission to install. Proceed and then it will be installed under you home directory. Once succesfully installed OFF will turn to ON
But if you check gnome tweak tool, most probably, shell theme will be seen disabled. The fix is mentioned in the comments section of the page.
sudo cp $HOME/.local/share/gnome-shell/extensions/user-theme\@gnome-shell-extensions.gcampax.github.com/schemas/org.gnome.shell.extensions.user-theme.gschema.xml /usr/share/glib-2.0/schemas/ sudo glib-compile-schemas /usr/share/glib-2.0/schemas
Now check gnome tweak tool, you will be able to change the shell theme.