The first version of TechSpot dark mode dates back to mid 2018 as an internal side project we've taken up and left on the back burner twice now. Content has taken precedence (as it should!), but the good news is that we have a near final revision almost ready.
The latest WhatsApp Beta (version 2.20.13) finally brings the long awaited dark mode. To activate download here, install and head to the three dots in the upper right corner. Next select Settings > Chats > Theme > Dark. FYI, going from stable to beta doesn't affect your chats or settings in anyway.