Google Duo is the latest Android app to receive dark mode treatment. The change was done through a server-side switch, so there's nothing for you to do. The colour scheme is similar to the grey shade used by other Google apps, and it follows the system-wide dark mode for Android 10. But there's also a way to set the theme manually, which means it's possible older versions of Android might get it, too. For now, the feature is visible on Duo v63.
Source: Android Police