Complete section self-link contrast fix in dark mode

[Copilot]

Accessibility reports flagged insufficient contrast for the § section self-link icon; while the light-theme opacity update was already handled, dark mode still rendered some section icons effectively invisible. This change closes that remaining dark-mode contrast gap without altering existing light-mode behavior.

• Dark-mode self-link color override

  • Added dark-mode CSS overrides for h4/h5/h6 self-link glyphs so they no longer use hardcoded black on dark backgrounds.
  • In dark contexts (prefers-color-scheme: dark and ReSpec’s explicit dark toggle), the icon now uses var(--heading-text).

• Coverage in style unit test

  • Extended the existing core style unit test to assert both dark-mode rules are present in injected ReSpec CSS, preventing regression.

@media (prefers-color-scheme: dark) {
  :is(h4, h5, h6) + a.self-link::before {
    color: var(--heading-text);
  }
}

body:has(input[name='color-scheme'][value='dark']:checked)
  :is(h4, h5, h6) + a.self-link::before {
  color: var(--heading-text);
}

• Fixes Accessibility improvement : section icon has insufficient contrast #5338