WhatsApp is working on a dark mode for its popular chat app. That claims the site WABetainfo. It would not be the first big app to introduce such a mode, recently it happened for YouTube on Android.
A darker mode can be found in more and more apps; one of the most recent additions to the feature was in the YouTube app for Android. MacOS 10.14 and the Outlook site also fall under recent additions. WhatsApp alternative Telegram has been using a night mode for a while, which uses a dark color palette with white text.