The features are contained in the most recent update of the Android beta version of the app - 2.20.31 - available through the Google Play Beta Programme.
WhatsApp enabled the Dark Theme in the 2.20.13 beta and has been tweaking the option in subsequent updates.
The solid colours revealed in the latest version will be available as wallpapers, the report stated. The colour options are black, dark brown, dark navy, dark olive, dark purple, and dark velvet.
WABetaInfo said that if a user selects a dark solid colour as a wallpaper and changes the theme later, the wallpaper will automatically revert to a lighter version of the colour.
It also noted that WhatsApp had removed a Battery Saver selection from the list of theme options in a previous update. Users can now only choose between Light and Dark.
It is not yet clear when WhatsApp will officially roll out dark mode to the general public. Reports about the feature's development first surfaced almost a year and a half ago, in September 2018.
Many other mobile platform and app developers have rolled out dark modes in their services, including Apple and Google. Facebook's Instagram app already offers a dark mode, but the feature still needs to be added to Facebook itself.
