Ubuntu design gathered the near future multimonitor objectives in an in-depth Google documents explaining step-by-step the steps to be addressed starting with Precise Pangolin, but, a definitely interesting aspect of the documents, is represented by a "leak" of the possible new GTK+ theme.
This may be just a mockup, the next theme or, as shown in the above image, an enhanced Ambiance.
The buttons are thicker and have rounded corners, roundness that can be observed all over the window.
Buttons are whiter, cleaner and the search icon is different too.