Can someone give me an example how to use an icon from set in a scripted GUI (JS), if possible? Thanks.
I'm not sure they can. I can't find anything in the manual about script dialogs using Icon Sets.
Scripts can be bundled with standalone icons (e.g. a PNG file) which can be loaded into dialogs. They can also add Icon Sets for use on Opus toolbars. But I don't think they can use Icon Sets (whether their own or installed standalone) in their dialogs, unless it's not documented.