#myBtn {
 display: none;
 position: fixed;
 z-index: 99;
 border: none;
 outline: none;
 background-color:#50c878;
 color: black;
 cursor: pointer;
 border-radius: 30px;
}

#myBtn:hover {
 background-color: #555;
}
@media only screen and (min-width : 790px) {
  #myBtn {
    bottom: 20px;
    right: 30px;
    font-size: 18px;
    padding: 12px;
  }
}
@media only screen and (max-width : 789px) {
  #myBtn {
    bottom:15px;
    right:15px;
    font-size:14px;
    padding:8px;
  }
}