Comment 3 for bug 1478173

Note that *actual tooltips* are fine, since they render with the background-image, as specified in gtk-widget.css.

But firefox's notification bars have trouble because they don't get rendered with the full GTK styling. Firefox sets up a dummy widget and styles it as a tooltip, and reads its "background-color" and "color" styling, and assumes that those will be useful. And the problem is that background-color does not currently have a useful value (in that it's basically the same as the foreground color), because these GTK themes reasonably don't expect that they need to provide a background-color, since they're providing a background-image instead.