You can look into using the Filtered Topic Lists theme component with the in:pinned
query.
If you instead want to stick to CSS, you can do something like this:
// target the last pin in the list
tr.topic-list-item.pinned:has(+ tr:not(.pinned)) td {
// rough way to force space inside a table row
padding-bottom: 1.8em;
}
tr.topic-list-item.pinned:has(+ tr:not(.pinned))::after {
// content to display in the space if wanted
content: 'regular';
width: 100%;
position: absolute;
display: flex;
height: 1em;
left: 0;
right: 0;
bottom: 0;
justify-content: center;
border-top: 1px solid var(--primary-low);
}