Many comments here say that that's what browser says but there is no way to distinguish UI theme from content theme preferences and it's impossible to have dark UI with light content due to browser limitation. Hence, as they say, it's a problem of a browser, not a web-site. If it's obvious to everybody that these content preference signals from a browser lack crucial information as to distinguishing between UI and content preferences, maybe it's safer to keep standard default (usually light) and just offer and option to switch to dark theme in case a user wants and remember preferences locally till the next visit.

There is no need to force dark theme in the absence of enough information from a browser as to exact user preferences. Browser doesn't ask for anything. It just says what UI theme a user has. It's a web-site that makes a conclusion based on limited information and then decides to force dark theme on content.

Hence, my post here is to show this situation.