Q:

Changing the layout of a navigation bar on wordpress

I created a nav bar and was wondering how I could edit the CSS of it on wordpress and by that I mean calling the nav bars by it's name( from php to css I suppose). 
As it's just two dots showing variables. 
This is what I have as nav bar.(It works even though there are no variables there). 
    function register_my_menu(){
    register_nav_menu('header-menu',__( 'Test' ));
}
add_action( 'init', 'register_my_menu');

I have no idea how I should edit this in the style.css as I am new to wordpress but have common knowledge on php/css yet this keeps me questioning all the time. 

A:

<html>
<body>

<ul>
   <li>1</li>
   <li>2</li>
</ul>

<ul>
   <li>10</li>
   <li>20</li>
</ul>

<style>
body ul:first-child {
    background: red;
}
</style>

</body>
</html>

