.boton-regresar {
        position: fixed;
        bottom: 20px;
        right: 20px;
        background-color: #333;
        color: #fff;
        width: 40px;
        height: 40px;
        border-radius: 50%;
        text-align: center;
        line-height: 40px;
        font-size: 20px;
        z-index: 9999;
        transition: opacity 0.3s ease;
    }
    .boton-regresar.hidden {
        display: none;
    }