.theme-responsive-menu-ancestor> a {
  /* Add your desired highlight styles here */
  color: #140F3B !important;
  /* background-color: #YOUR_HIGHLIGHT_BACKGROUND; */
  /* border-bottom: 2px solid #YOUR_HIGHLIGHT_COLOR; */
}

/* You may also need to target the li element itself */
.theme-responsive-menu-ancestor {
  /* Add background or border styles here */
}