Dark mode is now available for the WhatsApp desktop and web clients. To activate it, download the latest version (2.2027.10). Once installed, choose from the three dot menu and find the theme option in settings.
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.