Next time please ask it to respect system dark/light mode preference, it's trivial to do, especially for an LLM which can spin up light/dark alternatives easily.
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.
That's fine, too. Either way, give the user the choice.