Customizing Pandoc's Slidy CSS?

I'm using the amazing Pandoc to convert from Markdown to the Slidy slideshow format. I'd like to edit the Slidy CSS but I can't for the life of me figure out where the CSS file is that's getting used. I've found a bunch of slidy.css files here and there and tried modifying them, but Pandoc's Slidy output never changes.

Does anyone know where slidy.css is by default on OSX? Thanks!