* mediawiki-ui styles (i.e. Jared and May's styles)
MediaWiki UI is a transitional system which has allowed interfaces to be modernized and the design work to be iterated on, in context, before OOjs UI was ready for use. There shouldn't be any significant differences between MediaWiki UI and OOjs UI, but where there is a difference we should seek to make both systems look the same where possible until MediaWiki UI is no longer in use. OOjs UI within MediaWiki is using the theme designed by May, implemented by Prateek with some help from me. They are not my styles.
The new styles have not been back-ported to jQuery UI, and I don't believe there's any plan to do so. The objective is instead to stop using jQuery UI and replace those interfaces with their OOjs UI equivalents over time. Unfortunately jQuery UI is difficult to theme, which is actually one of the many reasons we have decided to move away from it.
* default MediaWiki element styling (skins)
Default MediaWiki styling (per skin) is really the lack of styling in practice because none of our skins provide styled form controls or other reusable widgets.