In macOS Mojave you can enable Dark Mode via System Preferences > General > Appearance > Light or Dark – but this is a one monolithic change for the macOS and any supporting Apps, what about changing the Dark Mode for separate unique Apps whilst keeping the macOS in Dark mode?
You can do so in the Terminal using an apps Bundle Identifier – this guide switches the Mail.app appearance from Dark to Light.
Get the macOS app Bundle Identifier
Launch the Terminal and find out Mail.app’s Bundle Identifier by entering…
osascript -e 'id of app "Mail"'
So you leave off the suffix .app and just enter the App name and include the quote marks as above. You will be returned…
Which is the Bundle Identifier
Change the App to Light Mode
defaults write com.apple.mail NSRequiresAquaSystemAppearance -bool yes
Then restart the App.
Change the App to back to Dark Mode
defaults delete com.apple.mail NSRequiresAquaSystemAppearance
To reset the App back to Dark mode use the above command.