ok. by scroll top you just mean hover... with sprites. and i dont mean to be an ass, but if that is what you are trying to achieve why havent you copied their markup?
function add_stuff() {
echo '<div id="back-to-top"><a href="#" rel="self" title="Take me to the top of this page">Back to top</a></div>';
}
add_action('thematic_header','add_stuff',6);
that wrapper div is not totally necessary, especially if you give the link an id
then css:
#back-to-top a {
background:url("images/shapely-back-to-top.png") no-repeat scroll center top transparent;
bottom:-57px;
display:block;
height:127px;
position:absolute;
right:40px;
text-indent:-9000px;
width:197px;
}
the important parts are display: block; so that text-indent:-9000px can throw the text so far off the left that you won't see it. and the position parts are important to essentially 'crop' the full image to just the sprite you want to use on the 'down' state.
then for the hover you change the background position to whatever is appropriate
#back-to-top a:hover {
background-position: center bottom;
}
i just lifted their css so it is obviously not going to be exact for you.