After that copy these files into your theme. In case you use default theme, it should be content/themes/casper/assets/js for the JavaScript file and content/themes/casper/assets/css for the style sheet file.
Modify content/themes/casper/default.hbs by adding following tags after respective comments and once you are done, upload your Ghost installation to your server.
{{! Styles'n'Scripts }}
<link rel="stylesheet" type="text/css" href="{{asset ""css/prism.css"}}"/>
{{!-- jQuery + Fitvids, which makes all video embeds responsive --}}
<script type="text/javascript" src="{{asset "js//prism.js"}}"></script>