Better implementation using a child theme:
Child theme’s functions.php file:
function graphene_scroll_to_top(){ ?>
<p id="scroll-to-top"><a href="#">=Scroll to top=</a></p>
<?php
}
add_action('graphene_developer', 'graphene_scroll_to_top');
And then style the link in CSS.
This way, you won’t have to override the entire footer.php file, which is not recommended cause I may add new codes to the theme’s footer.php file in the future.