WhatsApp Dark Mode Features
WhatsApp new upgrade on Dark theme discovers that WhatsApp can automatically enable/disable the Dark theme based on the system Battery saver settings. For instance, if a mobile changed to Battery saver mode then, WhatsApp automatically switches to Dark theme and after switching back to normal mode in system, again WhatsApp goes back with Light theme. However, the old system default option has been removed for unsupported Android versions like Android 9 & lower versions. Similarly, for Android 10 version supported with dark mode feature on default will works as system default in WhatsApp.
Likewise, Whatsapp changed the color of default profile picture Avatar background to black colour. So, after enabling the Dark theme mode, WhatsApp default profile picture Avator background image changed from White to Black.