The theme comes with the Natty wallpaper by default and if you use it in FC... then it'll look a bit awful :) since now FC15 looks like Natty. Anyway, according to the developer, this GTK theme supports Gnome tweak tool for switching between themes as well. Personally I don't like the colors that much, but that's just me. If you love those colors... then read-on.
How to install it?
First download Ubuntu Ambiance Gnome shell theme from here.
Here are the instructions from the creator himself...
Backup your theme directory first. Extract the zip file and rename "gnome-shell" directory to "theme" and copy to /usr/share/gnome-shell/. This path is usually where GNOME Shell is installed.Remember, it's a bit "evil" to make your non Ubuntu Linux Laptop/PC looks like Ubuntu (just kidding :P) Enjoy!.
Restart GNOME Shell by Alt, F2, then type r and press enter.