In the WhatsApp messenger, a dark theme will soon appear. It is reported that the hints for the early appearance of Dark Mode is already in the current version of the application.
A dark theme should come on both Android and iOS, which is even more relevant for the latter due to the two new smartphones iPhone with OLED screens.
When exactly in WhatsApp will add a dark theme, it is not yet clear. For example, in the last beta version of the messenger, it is not.