Viber is a calling and instant messaging application operated by a Japanese multinational company called Rakuten. Rakuten has been operating it since 2014. It is available on multiple platforms and can be downloaded from the App Store or Google Play for free for mobile.
Go into the application and go to "More" in the lower right corner of the screen.
Go to the "Settings" option in the “More” section.
From settings, go to "Appearance".
You will see the "Choose a Theme" option with a selection of three themes to choose from.
Touch "Black". Your application will enable dark mode.