Earlier some users reported that they were seeing the dark theme over their Chrome search page. It was last reported by users last month. Now reports say that Google is once again testing the dark mode in the search page. The dark mode does not turn the entire screen black rather makes it dark grey and text changes from black to white, whereas Google remains in its significant colors and links remain blue.
It is still in the A/B testing phase, so you might not counter the dark mode currently as it is available and limited to certain users. Google will directly match your preferred theme. It will identify your Mac or Windows theme and match itself to it. So if you are on the light theme you will see the usual google screen and if your system is on the dark mode, you will get the dark theme once you receive the update.