Add css_extras parameter to create_base_app. Legacy apps (blog, market
etc) get the default extras (basics.css, cards.css, blog-content.css,
prism.css, FontAwesome). SX app passes css_extras=[] — it uses CSSX
for styling and custom highlighting, not Prism/FA/Ghost.
Reduces <style id="sx-css"> from ~100KB+ of irrelevant CSS to ~5KB
of Tailwind resets + only the utility rules the page actually uses.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>