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.
What you want is light mode but to use a dark theme on your native UI. You can easy change that in GTK and QT and I remember it was possible to do in older versions of Windows. That will not change the prefers-color-scheme in your browser.
If you want your browser in light mode but the rest of your system in dark mode, that's a niche need, and not a preference you should be trying to force on the rest of the world.