Comment 4 for bug 817380

Seems to be resulting from themes like Ambiance (Ubuntu default) using white foreground color on black background and the tooltip style only setting the background color.

Can be fixed/worked around by setting the foreground color in a QSS file for each app (like Mumble, for which there are decent QSS files floating around the net). Which exact QSS rule applies to tooltips only remains unknown for the time being, but overkill approach of setting default style on * and then styling everything else does work.