Google is once again testing out Dark theme for its Search page. The last time users reported seeing a dark theme was last month.
Dark theme for the world's most visited page will follow your preferred Windows or macOS theme. If you've set yours to Light then Google Search will remain unchanged, set it to Dark and you'll be greeted with the new Dark theme.
Dark theme doesn't make the background completely black, but rather a very dark grey. Text has changed from black to white while links seemingly remain blue.
Google is still in the A/B testing phase so you may not have the change available just yet. Once you do, Google Search will greet you with a message about the new Dark theme with the option to enable it or go on as usual. There's no word when the Dark theme will carry over to your mobile browser's Search.
You could just change the background to solid black from bottom-right corner (customize this page).
In the last few months I got it several times with Firefox. I know it´s random, but I like it a lot.