Earlier this week, WhatsApp updated its app for both Android and iOS to now support native dark mode. On iOS dark mode is enabled or disabled system wide via the Settings app or Control Centre and on Android 9, the setting is within the WhatsApp app, under WhatsApp Settings > Chats > Theme > Dark
This isn’t a deep dark colour, it’s a very dark green colour, to match the WhatsApp theme.
“Dark mode for WhatsApp offers a fresh look on a familiar experience. It’s designed to reduce eye strain in low light environments. And we hope it helps prevent those awkward moments where your phone lights up the room.”
Source: WhatsApp Blog