found a very interesting bug that exists if you use nvidia driver and a gtk3 Version greater than gtk3.22 .
Just set a background image or gradient inside the gtk3 theme for .background.
While this should just enable you to set a background pattern or gradient for all windows this does cause the application to reset the bg color randomly inside menus.
An example of my problem is my oxygen scss theme on gnome-look.
Using nouveau or virtualbox graphic this error does not exists.
Also this bugs effects at least two different graphic cards and from 390 to 440. Did occur on two very different hardware.
kind regards bluedxca93