DiviThemeExamples

position:fixed;
width:100%;
top:0;
z-index:999;
// tutorial @ https://www.divithemeexamples.com/make-your-divi-4-0-header-sticky-or-fixed-with-the-theme-builder/

DiviThemeExamples

<script>
// When the user scrolls down 50px from the top of the document, resize the header's font size
window.onscroll = function() {scrollFunction()};
 
function scrollFunction() {
  if (document.body.scrollTop > 100 || document.documentElement.scrollTop > 100) {
    document.getElementById("myheader").style.position = "fixed";
  } else {
    document.getElementById("myheader").style.position = "";
  }

DiviThemeExamples

To create a child theme for Divi Theme (Elegant Themes) you will need the following 2 files 
style.css
functions.php
Divi Child Theme zip available at https://www.divithemeexamples.com/bare-bones-divi-child-theme/