EDIT: I just realized that I misunderstood this question, I though you were asking for help with CSS styling, not JavaScript selection. For that Stephan Olsen's answer is more apt. All of the selectors I reference in my post can be used with the querySelector method he references.

:nth-child(even) and :nth-child(odd) are real, valid selectors. So a CSS rule like this:

li:nth-child(even) {
    color: red;
}

Would make each even list-item element colored red.

As for selecting the third list-item specifically that would look like this:

li:nth-child(3) {
    color: blue;
}

Here is a JSFiddle showing them off. JSFiddle is quite useful for quickly playing around with CSS selectors and the like. So if you are confused or curious about something you just learned, then I'd recommend quickly setting up some HTML and CSS on that site and playing around with it.

I added even and odd colors to my list making app by making changes in the CSS file and index file.

In index.html I simply added a class to the <ul> of items

and in the style.css I added

ul.items, 
li:nth-child(even) {
  background-color: lightpurple;
}
ul.items, 
li:nth-child(odd) {
  background-color: lightsteelblue;
}

The reason I opted to things this is way is to limit the code I had to add in javascript, also adding a class to the ul tag is my choice in future proofing my project in case I want to make another set of list items.