Google is testing a dark mode for desktop search. Not all users are presented with the test yet and the dark version only becomes visible if the system used is also set to a dark mode.
9to5Google, among others, reports that there is an a/b test in which some users see Google Search in a dark version when opening an incognito window. Not only the homepage with the signature search bar is visible to some in dark mode; the same goes for the search results.
In this dark mode, a dark gray color is applied. According to 9to5Google, dark mode doesn’t work on all pages yet and may disappear again when a new incognito window is opened.
It seems that dark mode is equated with the chosen theme on the desktop at least in this test. This means that users do not have to manually turn on the dark mode with Search.
Google has confirmed to The Verge that there is a test, although it is not clear how big this test was. It is also unknown whether and if so when the dark mode will be definitively available for all users.