Disable Mix Blend Mode when Hamburger Menu opens in Header

Hello, I am trying to code the header to be mix blend mode. I have found a way to do so, but it effects the hamburger menu as well. I want to remove the mix blend mode on the hamburger menu when it is opened. And when it is closed, mix blend mode will enable again.
I am also interested in any other ways to customize specific target elements like the header-title-text to be mix blend mode, but I cannot figure it out.

I have been trying to accomplish this for 2 years and I am starting to get tired of trying. Can anyone help me?

Code below of what I have so far

<script>
function toggleheaderburger() {
  const header = document.getElementById('header');
  const headerburger = document.getElementById('header-burger');
  const headerburgerbtn = document.getElementById('header-burger-btn');

  if (headerburgerbtn.classList.contains('burger--active')) {
    // Hamburger menu is opened, set blend mode to "normal"
    header.style.mixBlendMode = 'normal';
    headerburgerbtn.classList.remove('burger--active');
    headerburger.classList.remove('header-burger');
  } else {
    // Hamburger menu is closed, set blend mode to "difference"
    header.style.mixBlendMode = 'difference';
    headerburgerbtn.classList.add('burger--active');
    headerburger.classList.add('header-burger');
  }
}

</script>

When burger menu is open - closes, body has a class name

Quote

body.header--menu-open

and

Quote

body:not(.header--menu-open)

you can use CSS code to target these to disable mix blend mode when burger is open

Doesn't seem to work, can you help me understand what to do with this code you have shared? Thank you for responding 

@KevinWalton can you share a link to your page?

Website is https://kevinwalton-portfolio-2023.squarespace.com

Password is: 1

@KevinWalton I only see mix-blend-mode attached to that custom mouse cursor, is that right? It doesn't look like it's attached to the header. If you've disabled your code can you put it back?

Sorry my bad, I was doing some editing. The code originally posted is now back on the site 

THANK YOU. This works!

Seriously I can't thank you guys enough. I have been trying to figure this out for over 2 years.

Thank you so much jaeveedee and tuanphan