A beta update for the app released on 26 March and contained the first mentions of the feature. A more recent update now includes more information on the dark mode option.
The latest update shows that WhatsApp is implementing an optional dark mode interface across its settings pages, as well as its Status Bar and Profile menus.
The feature cannot be enabled yet and WhatsApp is still working to add dark mode support for various menus and its chat interface.
There is currently no timeline for the release of WhatsApp's dark mode functionality.