Hello
The border on the product list page (Workshops) is black, but it only has 30% opacity which means that it looks grey. You should be able to fix this by setting the opacity to 1 (100%). You can do this by adding the following to Design > Custom CSS:
.tweak-products-category-title .products.collection-content-wrapper .nested-category-title.nested-category-title-padding::after {
opacity: 1;
}
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