I got the problem starting gnome-tweak-tool in gnome-shell 3.6 but can start in fallback mode.
After googling and found the solution as below:
sudo cp /home/USERNAME/.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
Where USERNAME is login name.
