In a dgml file, an expanded Node can have the Background color set which changes the background of the border and title bar. The Foreground color sets the color of the title text.
The body currently seems to be filled with a color that depends on the current color theme selected in the Visual Studio options dialog. This means some people end up with white backgrounds and some black. This causes issues, especially when trying to export as xps to print the diagram. A massive block of black ink is a waste of ink!
How can the color of the body of the expanded Node be specified?
No it cannot. It was a "UI design" decision. Filling the entire group background with color conflicted too much with the VS color scheme. For example, a dark theme editor showing tons of White looks ugly, and a light themed VS showing tons of dark colors also looks ugly...sorry.