Иван

@media (max-width: 767px) {
  .header_burger {
    display: block;
    position: relative;
    flex: 0 0 30px;
    height: 20px;
  }
....

.header_burger:before,
  .header_burger:after {
    countent: "";
    background-color: black;
    position: absolute;
    width: 100%;
    height: 3px;
    left: 0;
  }