In the picture is shown what works and what doesn't work (also doesn't work for warning descriptions in the code).
Current version Gnome 3.14.1. I tried to change window style settings with gnome-tweak-tool, but it did not change the looks. What else could I try? It worked fine with Debian Wheezy, e.g. Gnome 3.4 (and probably some days ago, before installing the many every-day upgrades coming with Debian testing. -> I'm going to check the aptitude logs.)