This add-on allows categories to be collapsible. Credit goes to @Steve F for pointing out the HTML/CSS codes along with the know-how. My work was to make it into an add-on to save the headache of modifying the codes manually.
unzip ALMUSA-CollapsibleCatergory.zip and upload to src/addons. In the Admin PC>Add-on find CollapsibleCatergory and hit install.