osascript -e 'id of app "Outlook"'

com.microsoft.Outlook

# Exclude from dark mode:
defaults write <Bundle-Identifier> NSRequiresAquaSystemAppearance -bool yes

# Dark mode on:
defaults write com.microsoft.Outlook NSRequiresAquaSystemAppearance -bool no

# Reset mode:
defaults delete <Bundle-Identifier> NSRequiresAquaSystemAppearance