I just realized that Google now allows dark mode. After googling for a while, I read that on Windows 10 if you have dark mode enabled, it will ask to switch or apply the dark theme by default. If not, you, theoretically you can enable it from Search Settings / Appearance.
But I still see the light theme and the "Appearance" option is not shown for me:
I have tried with a different account, with the same browser on the same PC and the "Appearance" menu is, indeed, shown. I have Windows 10 with dark mode enabled at a OS level.
How could I activate this option on Google Search settings for my other account?