Show the rendered HTML markdown to the right of the current editor using ctrl-shift-m.
It is currently enabled for .markdown, .md, .mdown, .mkd, .mkdown, .ron, and .txt files.
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;
}