User blog comment:BertH/Fluid layout has arrived/@comment-4237253-20131209072431/@comment-5556688-20131209131610

I agree that the layouts have been ruined by this update that I wish was optional. Nevertheless, I think the white space you refer to is because the background has been split in two to make the fluid layout fit. A solution would be to put in a color (through the Theme Designer) or make the background image so large that it will accommodate most 1920+ width resolutions.