Hi!
Happy to make my first contribution to this great community. I tried all of the CSS presented in this thread without any luck. It allowed me to alter sizes of the placeholders etc, but did not change the actual icons. So I went digging using the inspector tool in Chrome and found that I need to add this to my custom CSS to shrink the icon size:
.categoryicons .course_category_tree i.fa{
font-size:24px;
}
After that it worked like a charm. Hope someone else will find this useful!
//Oskar