Code line number have dark background when printed

Hi, Thanks for the plugin I'm using it often to preview readme files and write docs.

When printing a page rendered with markdownReader the line numbers have a dark background which I think is due to the box-shadow added at prettify.css:15. Adding the following removes the shadow for print and cleans things up:

    @media print{
        .prettyprint.linenums {
            -webkit-box-shadow: none;
            -moz-box-shadow:    none;
            box-shadow:         none; 
        }
    }

Would you consider adding to markdownreader.css or prettify.css?

With box-shadow: ![linenums](https://f.cloud.github.com/assets/600349/2311750/4dc33e3e-a2f4-11e3-80b7-89287bbd0efc.png)