The css page and rules that have been mentioned are the ones that are responsible for the side menu that you see. If you make edits to these styles you will update the css that is used.
CSS works with precedence, the last rule to be defined is the one that will be used. So if you have the same style defined in two different files it is the latter that will be used, this is one of the basics of css. This concept is discussed toward the bottom of this w3schools page on css.
To avoid confusion about the use of the other rules I made sure to specify only the ones that are relevant and resulting in the current state of the navigation. If you alter these styles and classes you will alter the look and feel of the menu.
Please post back if there is a specific styling that you are trying to add or edit that is not working out and I will gladly help you with whatever I can.