How is that going to work? User arrives at site, OS is set to dark, site is toggled to dark, so far, so good (assuming the toggle is smart enough to be set appropriately at this point).

1) User toggles OS to light, is your site going to switch back?

2) User toggles site to light, and back to dark to see what that does, now there's a persisted app setting. User later realizes they don’t like dark, toggles OS to light, and then has to think about your app's unique way of toggling it back.

To me, that's just unwieldy for very little gain. Why would I want your app to be inconsistent with everything else, other than a poorly executed dark mode?

While there’s a good chance you won't be using my current apps, I’m confident I won’t be the only one to reach a similar conclusion, so you'll likely have to get used to the behavior.

Your name here seems quite appropriate, as that was a weasel tactic, rephrasing the opponent's words to modify their intent. Not to mention you also used a quite inflammatory tone towards someone who's strongest words were "I strongly disagree".

I never saw any assertion that the user must manage apps individually, only that they should be able to. Easy way to do that - by default, the app echoes the system setting. If and only if the user manually changes the setting, does it diverge from that. Somewhat simple way around having a persistent setting get stuck - if the user switches back from the opposite mode, then you go back into "system" mode. So really, instead of needing two toggles, you would have one that has two states - "Respect system setting" and "Use the opposite mode from the system" (which probably need to be worded way better). The only snafu there that I can think of, off the top of my head, would be that if the user changes the system setting, the app state would likewise change.

It's also completely valid if someone doesn't wish to use system-wide dark mode (say, due to specific applications they use that may not function or render correctly in dark mode) but yet wishes to have specific apps in dark mode. I run Windows, which does have a dark mode nowadays, but that only works for certain apps that specifically use it (Windows Explorer does, not that I use that, along with many of the "modern" style apps that ship with Windows), and yet in apps like Discord I still prefer to use the dark mode, and I also have a dark mode extension for my browser.

For what it's worth, the site did correctly detect my dark mode setting in Windows, and display the dark style. As well, I don't necessarily disagree with any developer that does wish to exclusively use the system setting. However, I do see room for both approaches.

After some more thought, I’ve realized that should you want to allow but never force a user to manage appearance separately, you could give the toggle three options, ‘System setting’ (default), ‘Light’, ‘Dark’.