Nobody cares that a browser's navigation buttons, address bar, tabs, or window controls don't match the current website. Probably because these things are obviously outside the extents of the web page.

However, scrollbars, context menus, modal windows, and date pickers are rendered within the extents of the web page, and get replaced all the time.

It is my opinion that these controls don't need to be styled to match the website, because they're not part of the website. They're part of the browser. Non-diegetic. Outside the fourth wall.