Markdown Preview package 
Show the rendered HTML markdown to the right of the current editor using
ctrl-shift-m.
It can be activated from the editor using the ctrl-shift-m key-binding and is
currently enabled for .markdown, .md, .mdown, .mkd, .mkdown, .ron, and .txt files.
Customize
By default Markdown Preview uses the colors of the active syntax theme. Enable
- Use GitHub.com style
in the package settings to make it look closer to how markdown files get rendered on github.com.
To customize even further, the styling can be overridden in your styles.less file. For example:
.markdown-preview.markdown-preview {
background-color: #444;
}