no

considering free windows being light theme only, it should be a button, not a "system default"

By "free windows" do you just mean an unactivated copy of Windows? That doesn't prevent the user from configuring their preference in the browser itself.

There should be a button too, but it's simple to add a line so that it also defaults to any provided preference.

… is that even legal to do for microsoft? Are there no requirements to adhere to certain standards? Would have thought that is part of it.

That's fine, too. Either way, give the user the choice.