Complex code customization request: add a toggle effect to a navigation

Hello,

another big request, I hope someone can help me, I will give points graciously :wink:

On https://campus.ichblogge.com/lektionen/3-gastgeber-blogs/ you see a navigation with hierarchy: parent category > category > lesson
To make it clear: http://screencast.com/t/WAnq699AQjwT

All CATEGORIES have 2 actions:
1) they link to the respective category page
2) they can be toggled (by clicking the small toggle icon)

I would like to DISABLE THE LINK of the ALL categories in that navigation.
Instead, the toggle effect should not just be on the small icon, but on the whole category (navigation button).

To summ it up: clicking a category should toggle (open/close), clicking a lesson should link to the lesson.

Is this even possible with CSS?
I hope someone has a word of coding-wisdom for me :wink: