In short: Google appears to be testing a new dark mode for the Google Feed that you will find to the left of the home screen in the Pixel Launcher (and on some other smartphones). According to a user on reddit, this dark version of the Google Feed appeared after installing the September Security update from Google. There doesn't seem to be any settings available for changing between dark and light mode either. We have attempted to replicate this on several Pixel devices here running the September update, and none of them have a Dark Mode for the Google Feed.
Background: The Google Feed is part of Android, formerly Google Now, and provides all sorts of information at a glance. This feed will show you things like the weather, scores for teams you follow, some news articles it thinks you might be interested in, and so much more. You can really gather a ton of information from this feed, without having to jump into other apps to get the same information. It was previously (and still is for most people) available only in a light mode, which does look pretty clean. But a dark mode is definitely a better option for reading at night and such.
The Impact: While this is still being tested by Google, apparently, the change hasn't rolled out to everyone just yet. This is likely something that will be coming soon and could be part of the system's dark and light theme that Google debuted with Android Pie just last month. Having a dark theme would make it easier to read at night, though it does seem that it needs some critiques still. Seeing as the weather in the screenshot below is pretty tough to read, since it's black font on a dark gray background. But that is something Google will (hopefully) fix before it is rolled out in earnest.