Thanks to WABetaInfo, we know that WhatsApp users who belong to the app's Google Play Beta Program are starting to see the new Dark Theme inside version 2.20.13. Although the Program isn't currently accepting new signups, users can download the APK separately to enable the Dark Theme option. Once installed, it's a simple case of navigating to WhatsApp Settings and then Chats to switch it on.
There are currently four options in the Display setting. The Light theme uses the typical black text on a white background, while the Dark theme allows you to manually switch that around. System default mimics the system-wide theme set on Android Q phones and switches between light and dark depending on the time of day. The final option -- Set by Battery Saver -- can change the theme depending on your battery settings.
WhatsApp has been the holdout of Facebook's three big standalone apps when it comes to dark mode. Messenger was the first to enable it back in early 2019, with Instagram following suit in October. Although the Dark Theme is currently limited to beta versions of the Android app, it probably won't be too long until it makes it to a public release -- and presumably iOS 13 as well.