Once T117781 is done, GlobalPreferences should follow suit.
OOUI is a UI library that we're beginning to use on MediaWiki special pages. Special:Preferences is being converted to use OOUI in T117781. Once that happens, we should also convert Special:GlobalPreferences to use OOUI.
To see OOUI in action, go to https://en.wikipedia.org/wiki/Special:Preferences?ooui=1