If your questin is “how to add” a responsive “hamburger” menu on smaller screens, no…you cannot paste in the .CSS from another theme. There is a lot more to do.
Here is how to add the necessary functions and files to make it happen. The codes explained in the tutiorial below are what I used to add a “responsive” menu to Legacy. Yes, the menu does adjust to the small screenm but it is “stacked”. I wanted the same look and funcion as Epik, so I used this.
https://wpbeaches.com/using-slick-responsive-menus-genesis-child-theme/