6.0.4 — dark-mode headers follow the accent, headings step properly

Fixed

• Card and modal headers were the wrong colour in dark mode. Both were a hardcoded #1a237e — the light theme's navy, left behind when the dark palette moved on — so a card header was the one piece of chrome that did not follow the accent: light-blue title bars on the sidebar and over code blocks, a stray navy one on every card and dialog. Both now read --retro-primary / --retro-primary-fg, so re-theming the accent re-themes them too.
• Headings were bold at nearly the same size all the way down. h5 and h6 were both 16px bold, so the last two levels of an outline could not be told apart, and h2 carried h1's panel and accent bar, which made every section open with a second title bar. The scale steps 30 / 24 / 20 / 18 / 16 / 14 now, h2 is ruled off with the bevelled hairline the framework already draws under a card header, and h6 is small caps. .retro-h1 … .retro-h6 match.

Reverting either is one declaration; see MIGRATION.md.

All five gates pass: 36 unit tests, 370 token pairs at AA, the stylesheet gate, 140 rendered-page checks, 20 keyboard loads.