Currently, only Special:EditWatchlist is center aligned, Special:Watchlist isn't.
There are various other places where the centering doesn't quite work as expected
For example banners see screenshot:
Probably for that we need to add a new div containing all notices (siteNotice, localNotice, mw-dismissable-notice) and set a bottom border on that with a -1px margin, or we need to put the header inside another div that is full width and apply the border to that.