In the July 2017 update of VSCode, the VSCode team released a style contribution point for markdown preview. Now there's a VS Code Github Style extension from them, use it instead :D

A light theme overwrite whether you're using VSCode dark/light theme.

Using BigstickCarpet's gist, which is based on GitHub Markdown Repo

Add the following lines to your settings.json.

{
  ... // other settings
  "markdown.styles": [
    "https://cdn.rawgit.com/BigstickCarpet/5d31c053d0b1d52389eb2723f7550907/raw/88e4538c0fc46e39a97f332b43546f92762a6bb6/github-markdown.css",
    "https://cdn.rawgit.com/Yukaii/vscode-markdown-github-css/master/github.css"
  ]
}

or download these two stylesheets and reference them locally:

1. BigstickCarpe's github css
2. github.css

Modify settings.json as follows:

{
  ... // other settings
  "markdown.styles": [ 
    "file:///PATH/TO/BigstickCarpe's github.css",
    "file:///PATH/TO/github.css"
  ]
}

Markdown sample from facebook/react

• BigstickCarpe's gists
• CSS in original VSC repo