plugin.css

.back-to-top {
    position: fixed;
    bottom: 25px;
    right: 25px;
    background: rgba(0, 0, 0, 0.5);
    width: 50px;
    height: 50px;
    display: block;
    text-decoration: none;
    -webkit-border-radius: 35px;
    -moz-border-radius: 35px;
    border-radius: 35px;
    display: none;
}
.back-to-top i {
    color: #fff;
    margin: 0;
    position: relative;
    left: 15px;
    top: 14px;
    font-size: 22px;
}
.back-to-top:hover {
    background: rgba(0, 0, 0, 0.9);
    cursor: pointer;	
}
.book.color-theme-1 .back-to-top {
    background: rgba(112, 66, 20, 0.5);
}
.book.color-theme-1 .back-to-top i {
    color: #f3eacb;
}
.book.color-theme-1 .back-to-top:hover {
    background: rgba(112, 66, 20, 0.9);
}
.book.color-theme-2 .back-to-top {
    background: rgba(189, 202, 219, 0.5);
}
.book.color-theme-2 .back-to-top i {
    color: #1C1F2B;
}
.book.color-theme-2 .back-to-top:hover {
    background: rgba(189, 202, 219, 0.9);
}

@media only screen 
  and (min-device-width: 320px) 
  and (max-device-width: 480px)
  and (-webkit-min-device-pixel-ratio: 2)
  and (orientation: portrait) {
	.back-to-top {
		bottom: 10px;
		right: 10px;
	}
}