How can I add a divider on a product details page?

Hi! 

In this example, I've used some code to change the background color of the header: 

https://crab.squarespace.com/store/p/stickers

.collection-type-products.view-item .header .header-announcement-bar-wrapper {
  background: black;
}

Does anyone know if it would be possible to add a divider effect? Like the image example I've provided?

If you have an image file like this, we can use CSS code to add it under your header