I'm using a dark GTK theme. For some reason Libre Office Writer has a white background (like I want), but Impress does not. I do remember I had a white background in Impress as well some installations ago, so I think it should be possible to get this. I'm not sure if it matters, but I'm using Xubuntu with Xfce 4.10.
I just upgraded Libre Office to version 4, but the problem remains the same.
Have you tried to turn off "Automatically detect high-contrast"?