The Prism CSS was hardcoded to prism-tomorrow (dark-only), so code
blocks stayed dark even when switching to Light or other non-dark themes.
- Add id='prism-theme' to the <link> element for runtime lookup
- In _applyTheme(), swap href between prism-tomorrow (dark) and
prism (light) based on resolved theme
- Skips DOM write when the target href is already active
Fixes#505