With this approach, there is no active switch. If the operating system supports Dark Mode, the CSS respects that global setting.
Like when I go into my iPhone’s settings and switch EVERYTHING to Dark Mode (Settings.app → Display & Brightness → Appearance). macOS has a similar setting. I don’t know where to find it on Android, Windows, or Linux.
If we wanted an active toggle a visitor can use, we’d have to use JavaScript instead of (or possibly in addition to) this media query.
You can see a demo of this by switching your phone from light to dark mode on twitter.com, they seem to use this method. I added it to my site just last week: islovely.co/posts