The dark mode has arrived to another Google-made app, Google Duo. First and foremost, let's just say that this switch is being done on the server side, so it doesn't require you to update the app via the Play Store.
This is a server-side switch, but a server-side switch which is visible in Google Duo v63. So, if you have that version of the app, you'll get a dark mode eventually, hopefully within a week.
This is actually something similar to what Google is doing with the Play Store and Gmail. Both of those apps have received a dark mode over the last couple of weeks, and in both cases, a server-side switch is pushing that feature to devices.
Google Duo dark mode resembles other, recent implementations
Speaking of which, Google Duo actually adopts a similar dark mode to other Google apps. It utilizes a dark gray background, and combines that with various different shades of gray, and a pure white color.
Google Duo also follows the system-wide dark mode setting in Android 10. So, if you have set a dark mode on a system level in Android 10, it will be automatically applied in Google Duo.
Just for reference, you can do the same in Android 9 Pie, but you need to enable the option via Developer Settings. In Android 10, it's available via system Settings.
Google Duo follows quite a few Google-made apps who had received dark mode lately. Gmail and Google Play Store are the latest ones to receive such a feature, prior to Google Duo.
Prior to those, Google pushed out this feature to YouTube, Google Keep aka Keep Notes, Calendar, Chrome, and a number of other applications.
Many Google-made apps feature a dark theme option at this point
Pretty much every major Google-made app has support for dark mode these days. Google started massively switching to dark mode earlier this year. The company was aiming to push out dark mode to every major app before Android 10 arrives.
Well, the company did not really manage to do that, per se, but it came close. Several apps were left hanging when Android 10 dropped, but Google picked up the pace since then.
Following the Mountain View giant, tons of developers pushed out dark mode to their apps as well. Instagram is actually a good example of that.
Facebook started pushing out a dark mode to Instagram Beta quite recently, and it will arrive to the stable version of the app. Facebook's dark mode is coming in the future as well, and those are just some examples.