Divi 4 Theme Builder Hack for Fixed Navigation Bars (Pt. II)

style.css

/**
 * THEME BUILDER HEADER FIX
 */
.child.et_fixed_nav.et_show_nav #main-header .et_pb_fullwidth_menu .et_pb_row,
.child.et_non_fixed_nav.et_show_nav #main-header .et_pb_fullwidth_menu .et_pb_row {
  margin-top: 0;
  margin-bottom: 0;
}

@media screen and (max-width: 980px) {
  .child.et_fixed_nav #main-header {
    position: fixed; /* Delete this if you don't want a fixed menu on mobile devices! */
  }
}

.child .et_pb_fullwidth_menu .et_pb_menu__search {
  padding: 0 2rem;
  background-color: #fff;
}

/* Fix the upper margin in the Template Builder */
.child header#main-header.et-fb-root-ancestor {
  top: 0 !important;
}

/* Fix the padding at the bottom in the Template Builder */
.child header#main-header #et_pb_root {
  padding-bottom: 0;
}