Android 10 based EMUI 10 update brings a new feature of system-level dark mode, which is support by WhatsApp like third party apps.
To enable the dark mode feature in your WhatsApp go through the steps below:
Step 1: Download and install WhatsApp from play store or from our application page.
Step 2: Open WhatsApp
Step 3: At the right corner of your screen, tap the three dots
Step 4: Go to the Settings.
Step 5: Tap chats.
Step 6: Tap Themes.
Step 7: Select Dark.
The above screenshots will help you to change the theme.