Google’s video call application, Duo, is the latest Android application to deploy a dark mode. It seems to have received dark mode via a server-side switch, as pointed out by Android Police. Although the app was updated a few days ago, a dark mode was not added to the calling application at that time.
The dark mode color palette has the same shades of gray that other Google apps use. It follows the Android 10 system settings for dark mode. However, there is also a way to set Theme manually, which means that it could also work with older versions of Android.
The dark mode is currently visible in ‘Duo v63’, but it may be possible to use it on older versions of the app because it is enabled through the server-side switch.