QHeaderView::section {
    background-color: red;
    font-size: 10px;
}

QHeaderView::section:checked {
    background-color: green;
    font-size: 20px;
    font-weight: bold;
}

QHeaderView::section:first {
    background-color: yellow;
    font-size: 20px;
    font-weight: normal;
}