The chat bubbles appear in a dark contrasting color whereas the wallpaper and other settings menu have an all-black background with slightly contrasting menu bars.
WhatsApp has also added several dark solid colored chat wallpapers.
WhatsApp for iOS supports a lighter dark theme as well
As highlighted by WABetaInfo, WhatsApp for iOS supports another lighter dark theme as well. You can test it by increasing the contrast in your iPhone Settings. In this second configuration, WhatsApp uses lighter dark colors for some of the UI elements.
The update also introduces advanced search mode for iOS users
WhatsApp's new iOS beta update also introduces a new advanced search mode. With this feature, you will be able to search specific messages or media files stored on your device such as web links, GIFs, audio, photos, documents, and videos.