3 lines border options for top header area.

or a little more elegant:

#top #header:not(.header-scrolled-full) #header_main {
  box-shadow: none;
}

#header_main:before {
  content: "";
  position: absolute;
  left: 0;
  top: calc(var(--enfold-header-height) - 150px);
  width: 100%;
  height: 200px; 
  background-image:url("data:image/svg+xml;base64,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");
  background-size: 100% 100%;
  transition: opacity 1s ease-in-out;
}

#top #header.header-scrolled #header_main:before {
  opacity: 0;
}

svg with more convex look.

if you like to use it as svg separator you had to use a svg file like that
https://webers-testseite.de/button-row/
copy paste the svg code from there.
Link to the svg: Here

_____
Incidentally, you should adjust the fixed header rules for responsive behaviour. ( See it on small screens and scrolling down)

that is why i first tried to give you a header solution.
Then you do not need to add that lines manually to every top section in your content. – and if you got a slider or a gird-row on top no extra solutions are necessary. – But there are different Problems – f.e. what to do when first section got a collered background.

The separator solution: As part of next section this should not come into conflict with your menu text. Or do you have on other installations an overlapping first section to header area ?
The problem wil be now – what to do if you got a grid-row or a slider on top.