Olá,
Terá de alterar esta parte:
/*****************/
/* Scrool to Top */
/*****************/
#back-top {position: fixed;bottom: 5px;right: 10px;}
#back-top a {width: 50px;display: block;text-align: center;font: 10px/20% Verdana, Arial, Helvetica, sans-serif;text-transform: uppercase;text-decoration: none;color: #bbb;-webkit-transition: 1s;-moz-transition: 1s;transition: 1s;}
#back-top a:hover {color: #000;}
#back-top span {width: 50px;height: 50px;display: block;margin-bottom: 7px;background: #ddd url(
http://sites.comunidades.net/imagens/up-arrow.png) no-repeat center center;-webkit-border-radius: 10px;-moz-border-radius: 10px;border-radius: 10px;-webkit-transition: 1s;-moz-transition: 1s;transition: 1s;}
#back-top a:hover span {background-color: #777;}
Para:
/*******************/
/* Scrool to Top */
/*******************/
#back-top {height: 0px; Font-size: 0px;color: transparent;}Abraços,