I had another look. The problem is specific to the Antiquity theme.
Apparently, the other themes (Century, Conservative/Liberal, etc.) are layered on top of a shared "base" stylesheet, with the theme's own stylesheet only describing the theme's differences from this base, but Antiquity does not use this base stylesheet and defines everything from scratch. The tr.thisisme rule was moved to the base stylesheet rather than the sheets for individual themes, so Antiquity is missing out.