Need Help in my Multilevel CSS Accordion Menu

I am trying to make a CSS multilevel Accordion, But I am having difficulty in creating a Sub Menu inside it. Can anybody tell me how how to add another level in it.
I want it like this.
Menu 1
Menu 1.1
Menu 1.1.1

Menu 2
Menu 2.1
Menu 2.1.1
Menu 2.1.2

Codepen link: