@Jeck11 It was a while ago, so I don't remember enough. As such, I'm not sure I'd go as far as saying "any" theme, but it wasn't particularly hard, no. I suspect it depends on how modularized the theme files are (i.e. if you have to add the icons to each of the system files, it can be a pain, where if it's all centralized in the theme.xml file, then it's fairly easy).
It's critical to update the XML version in the theme xml files, though, otherwise it won't be recognized no matter what you do :)
I'm happy to try and share my versions of the themes, if it helps. Let me know.