I needed to make some changes to the header of DIVI site and this guy gives a great start to finish example.
With WordPress themes it is always better to create a Child Theme when you can so when your theme has a update your time spent customizing the CSS isn't lost.
I stumbled onto the following page the other day that was a great help to get started. I was use to just writing my own, but this saved a bunch of time.
Working on a new WordPress site I really wanted to go deep with DIVI. I got stuck on an issue with the dropdown submenu items. They were horizontal instead of a standard one below the other.
The issue was quickly solved when I created a new menu and made that one the primary.
I was surprised I wasn't able to do this easier, but it looks like others have struggled with this as well. I did get it fixed, and was able to limit the word count. I didn't make a override yet but I will.