Beta versions of WhatsApp for Android often reveal the new features and improvements that are in development. The latest beta has now been submitted and version 2.19.82 reveals that a dark mode is being developed for the Android app. This feature is obviously not available for everyone just yet.
There has been sort of a dark mode awakening among app developers over the past couple of months. Many popular apps have been updated with support for the darker theme. Facebook-owned WhatsApp is evidently making a similar move.