#back_to_top {
    position: fixed;
    bottom: 40px;
    right: 40px;
    z-index: 9999;
    width: 30px;
    height: 30px;
    text-align: center;
    line-height: 30px;
    background: #f5f5f5;
    color: #444;
    cursor: pointer;
    border-radius: 15px;
    padding-top: 4px;
    display: none;
  }

  #back_to_top:hover {
    background: #e9ebec;
  }

  #back_to_top.back_to_top-show {
    display: block;
  }