WhatsApp is one of the last Facebook apps to get dark mode support on the Apple iPhone and the latest beta now includes that feature.
Unfortunately, in order to access that dark mode option, you need to be in on Apple's TestFlight platform, which is very hard to get into.
However, users on Reddit are reporting seeing the option to switch to the dark mode theme, so it's clear it is in the testing phase and will hopefully be rolling out soon.
A dark mode theme for WhatsApp has been a long time coming. Images for the update to Android leaked in early 2019 and even Facebook Messenger added the dark mode option around then too. We do wonder why it's taken so long for the dark theme to make its way to iOS.
That said Facebook's main app still hasn't got dark mode either, but that's in testing too.