When it lands on the Beta channel, I’ll be able to test. Technically, there’s no reason why it wouldn’t work, if ChromeOS (like Android, iOS, macOS, etc.) respects the prefer-color-scheme media query.
I just tried this out on a Chromebook in the beta channel. It kinda works, but it is unpolished. I could get it to force dark mode for all web content, including Discourse sites (I tested using try.discourse.org). But it doesn’t look like it can toggle the web content when toggling the OS between dark and light themes. And playing around with it, I ended up quite often in a hybrid state, where parts of the UI are using light mode and other parts dark mode.
For others who want to try this out, head over to chrome://flags, search for “Dark”, and enable “Dark/light mode of system UI” and “Force Dark Mode for Web Contents”.