Why is my heading darker than it used to be?

Until 1.4.33 the "Share this" heading was drawn at 65% of whatever colour your theme uses for text. On a theme with near-black body text that reads perfectly well; on one with grey body text it measured 3.77:1 over #444 and 2.85:1 over #666, where the accessibility standard asks for 4.5:1. Nothing in CSS can put a floor under a faded inherited colour, so the heading now uses your theme's text colour as it is, and stands apart from the body by its capitals, letter-spacing and weight instead. If you would rather it were faint, your theme or a custom stylesheet can set .trss__heading { opacity: .65 }.