I had to edit these files:
/usr/share/themes/Ambiance/gtk-3.0/settings.ini /usr/share/themes/Ambiance/gtk-3.0/gtk.css /usr/share/themes/Ambiance/gtk-2.0/gtkrc
(Addition: for Ubuntu 12.04, it seems youjust have to modify the file: /usr/share/themes/Ambiance/gtk-2.0/gtkrc , replacing the tooltip backround and foreground color, with the #000000 and the #f5f5b5 color, respectively)
You require root privileges to edit the files. Use
gksudo gedit to edit them.
tooltip in these files and you'll find the color definitions for the foreground and the background. I use
#000000 as foreground and
#f5f5b5 as background and now the tooltips in all applications are again readable. After changing the color values simply switch to some other theme and then back to Ambiance and the tooltip color is now fixed.
tooltip_bg_color: #f5feb5 或 #f2edbc
Here is the result: