I use grip to preview markdown documents locally. To make the
generated page looks like README on GitHub add such line into ~/.grip/settings.py:
STYLE_URLS = ['https://cdnjs.cloudflare.com/ajax/libs/github-markdown-css/5.1.0/github-markdown.min.css']
It’s a CDN URL for github-markdown-css. This library is a minimal amount of CSS to replicate the GitHub Markdown style.