If you choose to enable it, your WhatsApp interface will look pretty much the same, just... darker, obviously. The app's developers have specifically chosen colours that are close to the system defaults on both operating systems, while a few minor design tweaks will make sure all the important information continues to stand out.
Android 10 and iOS 13 users can use dark mode by enabling it in system settings. Users on Android 9 and below can select it via WhatsApp Settings > Chats > Theme > Dark. It's rolling out to everyone on the latest version of WhatsApp in the coming days.