Ambiance is the Gtk theme that comes by default in Ubuntu since version 10.04 and has received slight modifications with each release since then.
Andrea Cimitan has shared on Google+ a screenshot of another change that will be included in this beautiful Oneric theme, which will now have a black toolbar.
If you compare it with the current version, you can see the elegance that the new settings bring.
Well done to the Ubuntu design team . Now (in my humble opinion) they just need to work a little more on the icon set to better match the black tones.