WhatsApp recently updated Darkmode feature in WhatsApp Beta Version 2.20.14. Now, the latest update feature enabled with Solid color backgrounds in Darkmode theme. 
WhatsApp Solid Colors in Darkmode Theme

 

WhatsApp added few solid colors for chats background in darkmode theme. Currently, on the new WhatsApp Beta update V2.20.33 included six Dark solid colors: Black, Dark Brown, Dark Navy, Dark Olive, Dark Purple, and Dark Velvet

To change the background colors of WhatsApp in Background theme. Firstly, enable the dark theme in Theme Option then beneath tap on the Wallpapers & now select Solid Colors. Here choose the your background colors.