So I just visited an article on Windows Central website and it looks like this
https://www.windowscentral.com/windows-10-os-development-schedule-changes
This is Edge insider Version 80.0.315.0 (Official build) canary (64-bit)

however, it looks like this on Edge classic

Websites are detecting my current theme in Edge insider Canary which is really good, and show me the relevant style of their sites.
when I set the theme to Light in Edge insider settings, that website's theme also switches to white, without the need to refresh the page or restart the browser.
I hope more websites react to this.