Apple does not allow other browser engines or does not even allow most Safari features to other browsers...
My point is that people are really stretching the term "skin". Bookmark syncing, tab & window management, history management, security management etc. all are features of a browser, and they have a major impact on the UX of that browser. They are WHY there are still users of Firefox or Chrome on iOS! They're not "skin" features, which typically imply minor look & feel related items.