I got it to work with Terminal (ha, probably terminal is better dark), but in any case, this works, and you only have to restart Terminal
defaults write com.apple.Terminal.plist NSRequiresAquaSystemAppearance -bool YES
Thank you. This is getting better and better
Figured out how to get Mail only to Light Mode -- Mail runs in a container, so the prefs are in the container, and you need to disable system integrity protection to make the change, and you can then turn it back on.
defaults write ~/Library/Containers/com.apple.mail/Data/Library/Preferences/com.apple.mail.plist NSRequiresAquaSystemAppearance -bool YES