Thank you @christopher.amirian for the clarity.
We are going to have to live with limited functionality on the header for now.
Can you maybe guide me in the right direction to edit the Shop/Cart/Checkout pages?
Adding a custom header means the shop/cart/checkout pages creates a break in the page layout.
I can not find where this can be edited? the archive-product.php page? Or can this be done by CSS? I can not isolate the class to add a margin?