I looked at the sites you linked to and they have no related print styles. I believe that they use the default print settings for browsers. Thus, one of the big changes is the background images hidden by default. For CSS tricks, the reason it might look so different is because it uses media queries. Therefore, if you limit your browser to 800 pixels or less, you will see that the message list expands to the full width of the browser, as when printing a document.
However, in general, print styles are either set using the media tag in the existing style sheet:
@media print{ }
or links to the stylesheet specifically for printing:
<link rel="stylesheet" href="css/main.css" media="screen"> <link rel="stylesheet" href="css/print.css" media="print">
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