h1
{
    text-align: center;
}
tr
{
    border: 1px solid black;
}
.seccion
{
    margin: 20px 0px;
    padding: 10px;
    border-radius: 10px;
    border: 1px solid black;
}
.volver
{
    text-align:right;
    margin: 10px 0px;
}
.asignatura
{
    background-color: #DDDDDD;
}