Mobile-friendly menu

Hello,
I’ve managed to create my own child theme and edit it accordingly to my needs. Everything looks fine except for the way the menu looks on mobile devices.
I would like to make a dropdown menu that appears only on mobile devices but I don’t know how to do it. Can anyone help me please?

I googled my issue and found a couple articles that explain how to add a dropdown menu but, since I’ve already edited my styles.css file, I’m afraid I’ll mess my layout if I add more code.
Any help will be greatly appreciated.

Link to the blog