Dark themes come and go these days. For example, Android, iOS and Windows now have one dark mode, while most of the new applications that are released have a dark appearance.
As it seems, Google is preparing one dark mode for the search engine, in an effort to provide more consistency on devices that have a dark theme enabled.
The dark feature on Google.com is only available for the browser Google Chrome and comes with an experimental flag for now. Logically after the tests, it will be released for all mobile devices.
It remains to be seen if dark mode Google.com will reach desktops at some point. Of course, it will take some time for the tests to be completed, but apparently everything will darken sooner or later.