If you are a git user, you can install the theme by cloning the repo:
$ git clone https://github.com/getomni/prismjs.git
Download using the GitHub .zip download option and unzip them.
To use the theme, just include the prism-omni.css
file in your page. Example:
<!DOCTYPE html>
<html>
<head>
...
<link href="themes/prism-omni.css" rel="stylesheet" />
</head>
<body>
...
<script src="prism.js"></script>
</body>
</html>