Comment 8 for bug 703451

The window background color is the same as the window border color, and they are chosen to match because resizing the window (on a non-compositing WM) causes the background color to be drawn in the regions that haven't been painted yet. So when the colors match the bleeding is at least of a single color.

We tried to provide hooks to let people (like you guys!) improve the colors, because GTK sorta leaves us hanging otherwise:
http://code.google.com/p/chromium/wiki/LinuxGtkThemeIntegration
(Comes complete with suggested modifications for Ubuntu Ambiance.)