.btn_to_top {
 position: fixed;
 bottom: 20px;
 right: 20px;
 width: 50px;
 height: 50px;
 border-radius: 100%;
 background: #e0e0e0 url('../img/btn-up.svg') center no-repeat;
 background-size: 23px 13px;
 cursor: pointer;
 z-index: 99;
 -webkit-transition: 0.2s ease-in-out;
 -moz-transition: 0.2s ease-in-out;
 -o-transition: 0.2s ease-in-out;
 transition: 0.2s ease-in-out;
}

.btn_to_top.hidden {
 display: none;
}

.btn_to_top {
 animation: show_btn 0.4s ease-in-out;
}

@keyframes show_btn {
 0% {
  opacity: 0;
 }
 100% {
  opacity: 1;
 }
}

@media screen and (max-width:700px) {
 .btn_to_top {
  width: 30px;
  height: 30px;
  background-size: 16px 9px;
 }
}