https://wccftech.com/how-to/how-to-enable-whatsapp-dark-mode/

WhatsApp dark mode is finally happening and we can't be more excited. WhatsApp's latest beta with build number 2.20.13 adds a dedicated dark mode option on WhatsApp. If you're interested, you can get WhatsApp's dark mode running on your device right now. Let's dive in to see some more details on the operation.

Take note that WhatsApp's dark mode is only available on Android devices for now and iOS users might have to wait a while longer to receive the update. The feature is part of WhatsApp's beta right now and if you're enrolled in the beta program you should get the update automatically as it is already available on Google Play.