html, body {
background-image: linear-gradient(rgba(255, 255, 255, 0.5), rgba(0, 0, 0, 0.5)), url('fondo-pagina.jpg');
background-attachment: fixed;
background-position: center;
background-repeat: no-repeat;
background-size: cover;
margin: 0;
height: 100vh;
}

.header{
margin-bottom:40px;
text-align:center;
}

.img-footer{
width:500px; height:auto;
}
.img{
width:750px; height:auto;
}
.tabla{
margin:0 auto; background-color: rgba(255, 255, 255, 0.5); width:50%;
}

@media (max-width: 768px) {
.img{
width:100%; height:auto;
}
.img-footer{
width:100%; height:auto;
}
.tabla{
width:90%;
}
}

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

.contenedor-principal {
    display: flex;
    flex-direction: column;
    min-height: 100vh;
    padding:0;
}
.contenido-principal {
    flex: 1; 
}
.footer {
    background-color: rgba(0, 0, 0, 0.5);    
    color: white;
    text-align: center;
    padding: 1rem;
    box-shadow: 0 -10px 10px rgba(0, 0, 0, 0.3);
    margin-top:50px
}