WhatsApp is rolling out dark mode to beta users on Android9
The folks at Wabetainfo learned recently that the latest version of WhatsApp submitted by developers through the Google Play Beta Program contains one important new feature – the long-awaited dark theme.
In that case, you can back up your chat history if you think it's necessary, uninstall the app and then reinstall it. This way the app will download the most recent software from the server, so you'll most likely be able to start using the new dark mode.
The new Dark Theme option appears in the WhatsApp Settings / Chats. You have three choices under Theme: Light, Dark and Set by Battery Saver. If you choose the latter, WhatsApp will automatically switch between light and dark themes according to your Battery Saver settings.