How to install a gnome shell theme?

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http://askubuntu.com – I switched from Unity to Gnome 3.4 but I'd like to tweak my desktop a bit. I want to use a gnome shell theme but the gnome tweak tool won't allow me to modify the gnome shell themes. I've read somewhere that if I reinstall the gnome-shell then it would conflict with Unity and I wouldn't be able to safely use Unity anymore. Is there a way to change the theme without reinstalling the gnome-shell? (HowTos)