.img-portada{
    width: 70%;  margin-left:15%; margin-right:15% ; height: 50%
}

.tienda-button{
    width:400px;  margin-top: -20px; 
    margin-left: 45%;
}

@media (max-width: 400px) {
    .tienda-button{
        width:400px;  margin-top: -20px; 
        margin-left: 35%;
    }
        
  }