I believe since December 2020, Google has been testing a dark mode theme in the Google Search results. Now I am seeing Google test a toggle to turn on and off the dark mode theme.
I personally have not seen this test but Google is definitely testing this dark mode on a small segment of searchers. Januka Harshan shared on Twitter screen shots of this new control feature to turn on and off dark mode in Google search:
You can also (if you are in this beta) see a setting for "appearance" in search settings that offers "device default," "dark theme" or "light theme."
I assume Google will roll this out to more users in the future, so keep an eye out for it.
No, dark mode is not a ranking factor.
Forum discussion at Twitter.
Update: Here is another UI For it:
Google is testing New Toggle Icons for Light & Dark Mode..
— aakash singh (@aakashsingh5) May 23, 2021
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Have a look in action
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cc: @rustybrick @brodieseo @glenngabe @aleyda pic.twitter.com/QBbn5gJK52
Update: September 2021:
This was officially announced on September 9th to be rolled out over a few weeks.
I am seeing the dark theme option again in Google Search https://t.co/LwoujGn1Qo pic.twitter.com/4vssP4CwHI
— Barry Schwartz (@rustybrick) September 24, 2021