@roger64
Maybe that is what I was thinking of then.
...
i now rediscovered the unfixed error in calibre. It acts just like Sigil.

System styles are a nightmare. Quite ugly too IMHO. Maybe it would be nice to do like calibre and ship with a guaranteed good theme.
...
FWIW, Cinnamon desktop, with the default Adwaita theme. And building Sigil against Qt 5.4.2 does look a lot nicer.
EDIT: Only in a chroot for Arch @ 6/15/2015 w/ qt5.4.2
Using the official Qt build in /opt/Qt5.4.2 it still looks bad. Stinking themes.
GUI looks on linux have gone down the toilet recently. This stuff is supposed to just work.