Description: Emacs shows blue background-color with gtk3-3.20.x from gnome-unstable

Hi!

After upgrading to gtk3-3.20.x from gnome-unstable emacs shows a blue background in my case.
Changing the emacs theme reveals that the background colour under characters is the one provided by the theme
but the rest of the window background colour is different.

I've another machine running debian and with this same combination of emacs 24.5 + gtk 3.20.1 the emacs background color
is rendered as expected...so I don't know where the problem belongs to...emacs or gtk.

Additional info:
* package version(s)
emacs: 24.5-2
gtk3: 3.20.1-1
* config and/or log files etc.

In the attached image you see a blue background that should be the same colour as the menu bar background.


Steps to reproduce:
Enable gnome-unstable and install gtk3 from there. Then start emacs as usual.