I much prefer reading white text on a black background. Is it possible for me to configure Mail.app so that messages are displayed white-on-black instead of the other way around?
The more universal way to do this is to turn the entire screen opposite (White on Black).
System Preferences > Accessibility > Display > Invert Colors
You can assign a hotkey in System Preferences > Keyboard > Keyboard Shortcuts > Accessibility > Invert colors
Default shortcut is: ctrl + option + cmd + 8
Yup. Go to preferences -> Fonts & Colors -> click "Select" on "Message font" -> do your thing. :)