/* Кнопка вверх */
.btn-up {
    position: fixed;
    background: white;
    right: 40px;
    bottom: 100px;
    border-radius: 4px;
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    transition: opacity 0.3s ease-in-out;
    width: 50px;
    height: 40px;
    opacity: 0.9;
    box-shadow: 0px 0px 4px 0px gray;
}
.btn-up::before {
    content: "";
    width: 40px;
    height: 40px;
    background-size: 100% 100%;
    background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='16' height='16' fill='gray' viewBox='0 0 16 16'%3E%3Cpath fill-rule='evenodd' d='M7.646 4.646a.5.5 0 0 1 .708 0l6 6a.5.5 0 0 1-.708.708L8 5.707l-5.646 5.647a.5.5 0 0 1-.708-.708l6-6z'/%3E%3C/svg%3E");
}
.btn-up_hide {
    display: none;
}
.btn-up_hiding {
    opacity: 0;
}
@media (hover: hover) and (pointer: fine) {
    .btn-up:hover {
      background-color: whitesmoke;
    }
}