Dark mode isn’t working for visitors.
Sure, the editor at /ghost can be set to dark mode, but for visitors it is always in light mode. And since I can’t find a setting for it, I would expect it to be automatic.
Dark/light mode should be selected according to a user’s preference in their browser/OS, and in CSS (and in javascript, technically) this can be picked up using the prefers-color-scheme media query.
That toggle is just for the admin interface. It’s not supposed to control the frontend website.
Whether a theme has dark mode is theme-specific. Lots of paid themes either automatically detect or provide a toggle.
I have a now-outdated list of mostly-official themes and how to get them to do dark mode here: Adding a dark mode button to your Ghost blog
TL;DR: You can set Source to dark background, but it doesn’t auto detect. Dawn can autodetect (it’s an option in the design & branding settings). Several other official themes are pretty easy to convert. Most paid themes include dark mode, either auto-detect or slider or both.