Ok, here is an example. It puts a line above nodes whom are first generation children of the zoomed node. I just made it a 10 pixel red line, but you can customize that. Let me know if you need more help, and I can take a look.
.Node.is-currentRoot > .Node-children > .Node-outer {
border-top: 10px solid red;
}
If you are interested in CSS, I try to drop tips in this thread Dynalist Custom CSS for Dummies