Lots of people have been waiting for Whatsapp Dark Mode, and WhatsApp Dark Mode haven't arrived yet in Official version or in Whatsapp beta Version official.
There is an Excellent Trick, which is official, and you don't need download any third party Apps. Its Official.
Here is How you can Use Whatsapp in Dark Mode in Any Android devices.
First Uninstall the current version of your Whatsapp, Take a Chat Backup,if needed.
Then Download WhatsApp Dark Mode Version and install it. Download Link below
Open it and Set-up With your number
Now Go to your Phone Settings, Open Apps then Select Whatsapp and Click on Force Close
Now Go back to Whatsapp, And click on Settings, Your will See Themes Option
Select Dark, congratulations, Now Your Whatsapp is Officially Running in Dark Mode.
Whatsapp Dark Mode Download Link