The multi toggle responsive navigation on smaller screens is similar to nested accordions, the user touches primary level parent item to reveal list under it and when the screen is big enough vertical toggle menu changes to horizontal drop down navigation.
If you are making a custom Content Management System (CMS) then responsive, almost pure CSS, admin template is for you. It is built with Bootstrap and provide functionality and ease of use at the same time.
If your website have hundreds of categories with each having their own sub categories and so on so forth. Then this Responsive CSS Mega Dropdown Menu is for you.
Download Simple CSS Overlay menu.
Amplify is free full featured accelerated mobile pages (AMP) template. It is a simple HTML template for Jekyll and static website or blog.