IMO if a website supports both light & dark themes, it should _by default_ switch automatically. If the site wants to allow overriding that, that's fine, but I hate having to constantly go into the settings of sites to switch it to "Use System Default" instead of "Light theme". Especially when sites don't store the data very long, or it's a site like AzureDevOps/Jira/etc which is deployed across multiple instances an…
To add the counter: A website shold always have a manual control and not try to guess my timezone or preferences based around that. Since I typically use a device in well lit conditions and detest dark mode as it is, not having a manual switch for when things go wrong kicks off a bad experience.
To clarify, I was referring to it just respecting the OS's day/night cycle settings. I definitely agree a website shouldn't be trying to do anything fancy like that, I was just saying that websites by default should respect whatever the OS/browser is listing as the preference