function my_print_sheet() {
echo '<link type="text/css" media="print" href="' . THESIS_CUSTOM_FOLDER . '/print.css" rel="stylesheet" />';
}
add_action('wp_head', 'my_print_sheet');

This function assumes a custom print.css located in the custom folder.

Print.css reference for custom_functions.php in Thesis

php, wordpress, printing

PHP