I don't know of a way of doing it within ADE (either old or new version).
If you are on a recent Windows, try using Invert Colours option on the Magnifier feature.
I just tried it on Windows 8.1 and it worked fine.
It does invert colours on all windows, but probably not a problem while you are reading abook.
Easy to turn off and on again in any case.
Yes, I just downloaded ADE and was wondering the same. Why is this not an explicitly supported feature? Black background is easier on the eyes and more gentle for device or laptop battery life.