@kleinerpirat
There’s this PR which would certainly help you out.
Besides that, you can only access it with add-ons. A somewhat minimal example for the Python part would be load_collapsible_icon_js in Collapsible Fields. Of course, you’d only need the CSS file.
A minimal example for the JS/CSS would be the load function here in load. You’d need to adjust textContent (or load it through a link element with href).