This problem is related to the template styles. They override the Slogin styles.
The simplest thing to delete is this style.
.s5_responsive_mobile_sidebar_light #s5_responsive_mobile_sidebar_login_wrap span {
background: url(../../images/vertex/responsive/login_sidebar_light.png) left center no-repeat;
background-size: 19px 19px;
padding-left: 36px;
display: block;
}