Is it possible to add the_post_thumbnail to style.css?

If you want to add background image you can do inline css in html

PHP:

<?php
if(has_post_thumbnail())
{
    $desktop_src = wp_get_attachment_image_src(get_post_thumbnail_id(), 'desktop');
    $ipad_src = wp_get_attachment_image_src(get_post_thumbnail_id(), 'ipad');
    $mobile_src = wp_get_attachment_image_src(get_post_thumbnail_id(), 'mobile');

    $desktop = 'style="background: url('.$desktop_src[0].');"';
    $ipad = 'style="background: url('.$ipad_src[0].');"';
    $mobile="style="background: url(".$mobile_src[0].');"';

}
?>

HTML:

<div id="desktop" <?php echo $desktop; ?>></div>
<div id="ipad" <?php echo $ipad; ?>></div>
<div id="mobile" <?php echo $mobile; ?>></div>