I'm not too surprised by your discovery. When themes are enabled, there is simply not just one background color, but many (since it is a gradient).

I'm wondering why you are trying to get that color. I think that the best thing to do is to always assume that containers are transparent and whatever background color is visible is just what is underneath but it is not the real background color of the container.

As far as I know, GTK with theme, will probably have a similar problem where you cannot get the exact background color.

We spend a lot of time to ensure that Vision2 applications will follow the theme chosen by the user at the OS level. So changing colors of containers/widgets might not be the best thing to do on a UI level, but this is up to you to decide.