Primary menu bar bottom border like triangles or waves

I guess the easiest way is to create a background image for your shape / pattern & use custom CSS in Divi > Theme Options > Custom CSS to add it as the background image for your header:

Code:

Log In or Sign Up to view this code.

Not quite sure what you meant by this. Kindly elaborate more.

 

If you use CSS like this: https://stackoverflow.com/questions/13774952/making-jagged-triangle-border-in-css
**You need to set a fixed background color below the triangle, which is not desirable for most cases. This is because different pages usually have a different first section background color.

Not sure of any other easy way to achieve that.

You may try to ask official Divi support about it & see what they say: https://www.elegantthemes.com/members-area/help/