Assuming you want to install gnome shell themes by gnome tweak tool, here is what i do:
1. install both gnome-tweak-tool and user-theme gnome shell extension
2. download a theme tarball. gnome-look.org has a good collection of gnome shell themes. (i am using a slightly modded version of this:
http://gnome-look.org/content/show.p...content=143383)
3. make a directory .themes at home and extract the
theme folder (the one which has a gnome-shell.css file in it) at ~/.themes
4. reload gnome shell (alt+f2-->r and enter)
5. now you should be able to select and switch between gnome shell themes from gnome tweak tool