WhatsApp offers two ways in which you can enjoy the new dark theme. You can either manually enable dark mode for WhatsApp, or you can select the system default option to automatically match your phone's theme.
How to manually set dark mode in WhatsApp
Open WhatsApp and tap on the action overflow menu (three vertical dots icon) at the top right corner. Under this menu, click on Settings > Chats, and then Theme.
In the Choose Theme menu, select the Dark option to enable dark mode across the app.
If you want to switch to the light theme, follow the same steps and select Light instead of Dark.
You can also schedule dark mode based on system defaults
In case you prefer using dark mode only at night, you can use the System Default option to automatically switch between dark and light themes based on system settings.
However, for this option to work, make sure your phone supports a system-wide dark mode (available on Android 10). To enable it, you can set a time-range to automatically activate and deactivate the dark theme.
Not OLED friendly
WhatsApp dark theme won't help save battery
WhatsApp's dark mode imparts a dark green look to the entire UI with some contrasting color accents. In the chat menu, only the text bubbles are dark while the background remains user-defined.
The feature helps ease eye strain and allows for easy readability in low-light conditions but since it's not a true black background, it won't help conserve battery on smartphones with OLED displays.