Its been a long wait, but WhatsApp has finally delivered a full Dark Theme for WhatsApp for Android.
The feature is currently in testing with the latest release of the beta app, version 2.20.13.
You’ll be able to change the theme under a new Theme page in the settings menu where WhatsApp will present three options; Light, Dark and System Default which will respect whatever the system default currently is.
Dark Theme has not yet been officially announced by WhatsApp,
See screenshots by WABetaInfo of the feature below: