Recently I found a new CSS rule for class mw-first-heading being introduced, which has been applied to group0 and group1 sites (as today is Wednesday):

.mw-first-heading {
  border-color: #eaecf0;
}

One strange thing is that such CSS rule could not be found on Beta Cluster (runs the master branch of MediaWiki code), and I failed to found such rule via codesearch (for less file). In my point this makes it hard to distinguish content and title, and the original style looks better, so I suggest a revert for such modification.