There's no Manual Dark Mode toggle in Whatsapp settings. But if you Turn On System-wide Dark Mode in you phone settings, then Whatsapp also changes to dark mode. Same goes for Instagram.
Okay i haven't received any update to my WhatsApp in the last 2 days, and now suddenly opening the app gives me dark mode. I have system wide dark mode enabled, but i dont have the new Facebook font in the settings page either . Maybe a server side switch or a/b testing