#back-to-top {
position: fixed;
bottom: 40px;
right: 40px;
z-index: 9999;
width: 32px;
height: 32px;
text-align: center;
line-height: 30px;
background: #000;
color: #fff;
cursor: pointer;
border: 0;
border-radius: 2px;
text-decoration: none;
transition: opacity 0.2s ease-out;
opacity: 0;
}
#back-to-top:hover {
background: #555; color:#fff;
}
#back-to-top.show {
opacity: 1;
}