Though I would add this in case anyone else had the problem I had: I was using an image for my website background and did not want that background to appear on my "print my invoice", but I wanted the rest of the stylesheet coming through (for fonts, etc) so I instead added:
<style type="text/css">
body {background-color: #FFFFFF;}
body {background-image: none;}
body,td,th {color: #000000;}
</style>
In the same place as discussed earlier, (print_my_invoice.php, between <link rel="stylesheet" type="text/css" href="stylesheet.css">, and </head>). The "body {background-image: none;}" is the important part that takes off the background image.
New 
