[mediaqueries-5] Preference overrides interaction with Extension APIs

[lukewarlow]

Need to decide on and call out how this interacts with extension APIs such as https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/browserSettings/overrideContentColorScheme (or any theoretical future ones).

[lukewarlow]

I don't actually have answer off the top of my head to this one. My gut says the precedence should be OS < Browser < Site < Extension but that's not a strongly held belief, and this needs wider discussions.