Copying GitHub Markdown Style
For many, the clean look of Github pages, issues and comments defines how markdown should be rendered. Here’s a few ways of getting the same look in your projects.
Sindre Sorhus made a site that covers everything about GitHub-flavored markdown. The CSS to replicate the GitHub style can be found at the matching GitHub repository.
Dmitri Shuralyov wrote shurcool/github-flavored-markdown as a extension to the Black Friday markdown processor. It takes markdown and turns it into HTML that should be identical to what GitHub produces. It also includes a simple CSS file to replicate the GitHub look. It's a little less complete than the Sorhus version but also a lot simpler and easier to start with.
Roll your own with Bootstrap v4
The bootstrap front end framework gets you quite close to the GitHub style. No surprise, they are both written (or managed) by the same person, Mark Otto. Transforming the default bootstrap style to the github style is a great way of learning CSS and bootstrap.