I want to use themes from Gnome Art, but I can't find where to configure
Control/Window Border/Icons in Ubuntu 12.04.
In Gnome2 you could further customise a theme by selecting either Controls, Colors, Window Border, Icons. Under Colors-> "Selected Items", you could change the color for "selected items". This changed the Window's border color (of the active theme).
There didn't seem to be a way to do this using gnome-tweak-tool...
UPDATE: But now there does! The response to How do I change the colors of the Ambiance & Radiance themes? explains how to use gsettings to override a theme's color setting(s).
This is what I did to change the Window border color and the Window background color:
You can find the default settings for a theme in /gtk-3.0/gtk.css amd /gtk-3.0/settings.ini. Just look for the line that starts with gtk-color-scheme =.