No way that I can find.
A workaround is to use a square shape to cover the entire slide. Tricky to position perfectly, but doable. If you drag the edges instead of hte corners it will snap in place more easily. Once you've got one slide's the color you want, leave it and duplicate to avoid having to do it over and over again.
To make it work for slides with content place holders, select the shape and then use the Arrange tab to move it into the background.
You can even use some of the shape's properties, like boarders and opacity to increase the range of possible effects; like changing the tint of the standard backgrounds.
Best I could come up with, sorry.