Not a Windows user these days but I understand it's not possible to get the old Windows 2000 look anymore, though I'm sure you can still change color themes and appearance to some extent.
If you do use Aero, you can change the window chrome colour to something custom, which should catch out sites trying to fake windows. I don't think browsers provide any way to get the window chrome colour, though come to think of it, I'm fairly sure IE does/used to provide system colour names in CSS, so it might not be impossible... (If you recall the Win2000-era colour customization window, which IIRC you can still access in current versions of Windows, it's just hidden, you can set things like default window background colour, which traditionally in the Win2000 era was grey. So I guess the idea is that websites could use these system colours if you changed them to be consistent with the OS. Of course nobody really does this.)
Since this is part of the initial setup wizard, I think it would be pretty hard to fake a Windows 10 dialog from inside a web browser.
You can afford to lose the remaining 3%.
The accent color? If you buy a new Windows 10 machine off the shelf, after you enter your name, it asks what your favorite color is. I think blue is highlighted when that screen comes up, but you can't miss the opportunity, so 3% is a bit low for an estimate of how many people will change it.
A fake dialog won't know that my titlebars and buttons should be teal.