The iphone back button is styled with a pointed left side to mimic the native iphone back button. This styling relies on rendering a different HTML structure with an outer div and inner span and input elements, unlike the regular button rendering which renders just an input element. This means that the proper iphone styled back button will only be shown to actual iphones or browsers with the user agent set to iphone. If a different device changes the theme to iphone they will not see the proper iphone-styled back button, as the HTML will not be what's required for that style.
We should instead always render an outer div and inner span, whether or not the user agent is an iphone. This would make the proper style dependent only on the theme.
Bug
Major