Add a border beside each forum.

[Matthew2D]

How do add a border in the following places?

 

[Floris]

This is kinda how you can attempt to find out where is what in XenForo,
what I used was: border-right: 1px solid red; (and green for left)

The tree shows it's using classes nodeList, node, and forum.
So you could try adding this to extra.css in the style you're editing.

.nodeList .node .forum { border-right: 1px solid red; }

 

[Matthew2D]

Thanks. Where would I put the code?

 

[Andrej]

Thanks. Where would I put the code?

The tree shows it's using classes nodeList, node, and forum.
So you could try adding this to extra.css in the style you're editing.

.nodeList .node .forum { border-right: 1px solid red; }

 

[Matthew2D]

It sort of worked...but it doesn't look right.

 

[Floris]

Now you know how to figure out what to put there.
Chrome has "inspect element" when you right click, highlighting the elements in the rendered page as you hover over them. It will disclose the tree and that's a great way to find out what to use and play with in the extra.css template.

If it doesn't look right, extend the exception in extra.css you've made, perhaps play with positioning or different classes, your arrows were very broad and just pointed to the sides, that's what I used as example.
Sorry that I don't have more time to make it more specific and 100% for you personally. I do hope it helps to get you started and learn XenForo's structure a bit more.