This has been bugging me, so I had another look and noticed from the css file that you have added the unreleased (in terms of inclusion in the main distribution) IE 7 content fix:
This then lead me to look at your lib_min.js file which reveals that you have altered this - but the one you altered was from the main distriution and not the one that went with the fix in the post of this forum. The latter contains code that properly facilitates the correct operation of the css selector.
Because of this hybrid it is not surprising that your code does not work and therefore would not affect anybody else. So, everybody else, please do not panic!!!
If you were to attach the image you want as a down arrow and state for what version of Moodle, I would produce a quick special for you as soon as I could incorporating the IE fix.
Creator of Collapsed Topics.