.header-bottom::before,
.header-bottom::after {
    content: '';
    position: absolute;
    left: 0;
    width: 100%;
    height: 8px;
    background: #800000;
}

.header-bottom::before {
    border-radius: 0 0 30% 30% / 0 0 80% 80%; /* curva arriba */
}

.header-bottom::after {
    bottom: -8px;
    border-radius: 0 0 30% 30% / 0 0 80% 80%; /* curva hacia arriba */

}