I added the css class so that it is possible to style the link in the list that matches the current page. I tested it on https://www.cthreepo.com and it works as expected.
Someone gave the plugin a bad review. I did not not receive any complaints about it, so I don’t know why. Sometimes, the competition gives other plugins bad reviews. I don’t make any money off of these plugins (unless you BUY MY SCIENCE FICTION BOOK!!!!), so I am not real concerned other than I feel bad that someone hated the plugin that much.
If you use this plugin, please go to https://wordpress.org/extend/plugins/collapse-page-and-category-plugin/ and give it a few stars.