Adding navbar divider lines using "li li" (CSS)

Hi!

Here’s the site I’m working on: http://www.studioamdevelopment.com/
I’m working on a child theme of Twenty Twelve.

I added this to my CSS to create divider lines between the navbar menus:
li+li { border-left: 1px solid #000000 }

However, it applies a black left border to everything on the page that’s a list. After looking through the comments in the tutorial I used, I found this tip:

“You want to use the css child selector > for that.
If the top level UL has an id of #nav the code would be #nav > li + li. This targets only the direct children of #nav.”

So my question is: can anyone help me find the “CSS child selector” for my top navbar, so that the left borders disappear from drop-down items and other areas on the page? I’ve tried a ton of things and none work.

This is my first time in WordPress and first time making a child theme, so, *newwwwbie.* Assume I know nothing ; )

Thanks so much!!