Need some help with forum CSS styling

Hello guys!

I’m currently setting up my forum and I want to change some defaults.
I have SMF 2.0.2
The problem is that by default it doesn’t have a post separator. But there’s a mod to change that. Basically it modifies 2 files: Display.template.php and index.css
My question is about the latter one. It adds this at the end of the file:

.seperate {
/* border: 5px solid #fff; thickness and color of box outline*/
margin: 7px auto; /to push the gap between the posts/

I do get the white spacing between the posts, BUT what I had in mind was something that goes a little beyond that. I attached a file that shows how I would like it to look like more or less. The color of the border could be solid one, that’s not so critical to me.
When I tried to uncomment the 2nd line, what happened is that I got a border around the post and everywhere (ie. top, bottom, right and left) was the same. Same thickness that is…
Anyway, if anybody knows what to do please let me know!

It’s not really clear what to look at in the attachment. But you can set a different width for the border on each side, on just have a border on one side.