Hey bernieflyer, I have an A500, and after doing some checking around, it looks like you may be out of luck for the moment. Something about ICS forced Acer to remove the night mode. Folks over at xda have tried to install the apk from the honeycomb version, but the night mode won't work. Someone may figure out a way to mod the software to get it to work, or at least similar to the original night mode, but for right now, its either white or parchment background.
If you are interested in another app, I have read that "Aldiko Book Reader" has a great night mode, you may check that one out.
Hope this helps.