Make highlighted code readable on dark backgrounds
Make highlighted code readable on dark backgrounds

This works even with syntax highlighting because that's done client-side
and will override the stylesheet.

Fixes #781

marrero984/showoff,puppetlabs/showoff,marrero984/showoff,binford2k/showoff,binford2k/showoff,binford2k/showoff,puppetlabs/showoff,puppetlabs/showoff,binford2k/showoff,puppetlabs/showoff,marrero984/showoff,marrero984/showoff