#back-to-top-btn {
    display: none;
    position: fixed;
    bottom: 20px;
    right: 20px;
    font-size: 26px;
    width: 50px;
    height: 50px;
    background-color:var(--color-white);
    color:var(--color-white);
    cursor: pointer;
    outline: none;
    border: 3px solid var(--dark-blue);
    border-radius: 50%;
    transition-duration: 0.2s;
    transition-timing-function: ease-in-out;
    transition-property: background-color, color;
    z-index:20000;
  }

  #back-to-top-btn img {
    color:var(--light-blue);
    width:60%;
  }
  #back-to-top-btn:hover {
    background-color: var(--light-blue);
    color: var(--yellow);  
  }

  #back-to-top-btn:hover img, #back-to-top-btn:focus img {
    filter:opacity(0.4) color(0 0 0 white);
  }

  .btnEntrance {
    animation-duration: 0.5s;
    animation-fill-mode: both;
    animation-name: btnEntrance;
  }

  @keyframes btnEntrance {
    from {
      opacity: 0;
      transform: translate3d(0, 100%, 0);
    }
    to {
      opacity: 1;
      transform: translate3d(0, 0, 0);
    }
  }
  
  .btnExit {
    animation-duration: 0.25s;
    animation-fill-mode: both;  
    animation-name: btnExit;
  }

  /* fadeOutDown */
  @keyframes btnExit {
    from {
      opacity: 1;
    }
    to {
      opacity: 0;
      transform: translate3d(0, 100%, 0);
    }
  }