The Google App apparently did not have a proper dark theme until today. Google fixed that on both Android and iOS in an update or server-side switch that has begun rolling out this week.
The dark mode flips Google App results (Search) from light to dark, that’s basically the story here. If you are running Android 10, you can set it to follow your system theme setting, that way it’ll flip between themes as you do. Otherwise, you can force light or dark themes at all times.
To find settings for it, you’ll open the Google App, tap More in the bottom right corner>Settings>General and then look for the “Theme” option. Tap that and you’ll see choices of light, dark, or “System default.”
The dark theme could be tied to the latest Google App update which I can tell you that I received only a few short hours ago.
Google Play Link: Google App