fix: drop !important declarations from theme CSS (#5)

Clears the KB-CSS-001 warning from the knowledge base contract check.

- #topnav: both rules are unlayered with equal specificity; the branded
  one comes later, so it wins by source order without !important.
- [hidden]: moved Tailwind preflight's rule out of @layer base. Unlayered,
  it still beats every layered utility class without !important.

Co-authored-by: Oto Macenauer <oto@macenauer.net>
Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: 3 people