I don't understand this article. The author is using something that manipulates the style of inputs to make them dark with light fonts... but only if the input isn't explicitly styled with either background-color or color . If they're going to be doing such hackery, just style both at the same time. Use JavaScript or something to ensure both are styled. This really isn't the developer's concern.
> just style both at the same time
That's exactly the point. The system by default sets both colors, though most browsers I've used allow the website to override the colors. In these cases, the websites only override one but not the other.