Responsive menu

I'm trying to make the menu of a site responsive.
I have the main css and I added the media query to the end.
The site is:
It is a wordpress site. The theme's name is Magnificent.
What elements should I target to make this work? The color green is for testing.
The style.css:

Theme Name: Magnificent Child
Theme URI: <a href="
Version:" rel="nofollow">
Version:</a> 3.4
Description: 2 Column theme from Elegant Themes
Author: Elegant Themes
Template: Magnificent
Author URI: <a href="" rel="nofollow"></a>

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