go to qa-theme\Default\qa-styles.css
You need to change these lines:
.qa-favoriting {float:right; padding-right:48px; padding-top:4px;}
.qa-favorite-button,.qa-favorite-hover,.qa-unfavorite-button,.qa-unfavorite-hover {background:url(favorite-plus.gif) no-repeat; border:0; height:26px; width:26px;}
.qa-favorite-button {background-position: 0 -26px;}
.qa-favorite-hover,.qa-favorite-button:hover {background-position: 0 0px;}
.qa-unfavorite-button {background-position: 0 -52px;}
.qa-unfavorite-hover,.qa-unfavorite-button:hover {background-position: 0 0px;}