CSS class for collapsed categories

In a collapsible list, grouped by categories, I'd like to give a visual hint to the users as to "click here, and something will open". You have seen this: an arrow icon pointing to the right switches to an arrow icon point downwards as soon as the category link is clicked.

With CSS, this only works if the whole block has (or has not) a class="collapsed". As far as I can see, this only applies to the questions block (.faq-qa-hide collapsible collapsed - not to the surrounding category block.
Same might be helpful on the level below (question/answer).

If I were more of a coder and less of a designer, I would give it a try myself, but I guess the latter bugfixing might then use more effort ...