#btn-subir {
    position: fixed;
    bottom: 30px;
    right: 30px;
    z-index: 1000;
    background: var(--scroll-thumb);
    color: var(--blanco);
    border: none;
    border-radius: 50%;
    width: 48px;
    height: 48px;
    box-shadow: 0 2px 8px var(negro-65a);
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 22px;
    transition: background 0.2s, opacity 0.3s ease, visibility 0.3s ease;
    opacity: 0;
    visibility: hidden;
}

@media (hover: hover) and (pointer: fine) {
    #btn-subir:hover {
        background: var(--scroll-hover);
    }
}

#btn-subir.mostrar {
    opacity: 1;
    visibility: visible;
}