Google has changed the Dark Mode for many of its apps, such as Gmail and Store, and the latest to take this feature away from the company is the Duo video conferencing team.
When Dark Mode is changed, the Duet UI changes to dark gray, and the text changes to white. This reduces the strain of the eyes making it easier to use the app in low light periods.
Dark Mode in Duo should be around if you are on Android 10 then the whole system activates dark mode. However, you can also manually activate it from the application settings list, if you are on an older version of Android.
That said, Dark Mode on the Duo for Android will be enabled with a side-by-side switch, even if you are using the latest version of the application on your device, there is no guarantee that you will see this features from right now.