body > main > div:nth-child(4) > div > div > div > div > div > table > thead:before {
    content: '';
    position: absolute;
    top: 0;
    left: 50%; /* <-- put left edge in the middle */
    margin-left: -20%; /* <-- shift to the left by half of the width */
    width: 40%;
    height: 1px;
    background: #000;
}