Here’s How to Enable It, this is how to use WhatsApp Dark Mode on your phone

WhatsApp Dark Mode will give users the option to change the theme of the app, which will reduce the impact on the eyes. This mode will prove to be especially helpful in low light environments. Here we are telling you how to activate and use WhatsApp Dark Mode.
WhatsApp Dark Mode: How to enable it on Android

First of all go to Google Play Store and download and install the latest version of WhatsApp (2.20.64). After installing, tap on the three dots and go to the menu icon and select Settings. After this, tap on Chats and then go to Chat Settings. Now tap on Theme, here you have to choose the theme of the app. For dark mode, tap on ‘Dark’ and now you are ready to use WhatsApp Dark Mode.
You can also get a system default option when you tap on Theme. This automatically changes the theme to light and dark mode.
WhatsApp Dark Mode: How to enable it on iPhone

The manual method is not available for iOS 13 users. At the same time, users who do not have iOS 13 also cannot use WhatsApp dark mode.
To get dark mode on iPhone, go to the App Store. Now download the latest version of WhatsApp (2.20.30). Then go to Settings and tap on Display and Brightness option. Now tap on Dark to enable system. Alternatively, you can go to Control Center and then tap on Dark Mode.
The WhatsApp team has Blog I have put the complete details about this update.