For information only - since you have already got the file.
All the deb packages which you have downloaded should be still in your packages archive (unless you have been very diligent and have cleaned up your packages archive!)
Your packages archive is in
Thus for my version of gnome-shell (I'm using 32bit) I would extract the gnome-shell.css file directly from the archive deb file i.e.
dpkg --fsys-tarfile /var/cache/apt/archives/gnome-shell_3.2.1-0ubuntu1_i386.deb | tar -xf - ./usr/share/gnome-shell/theme/gnome-shell.css
i.e. you pass the path + file name as a parameter as shown (all paths are relative so its
./usr/... rather than