Customizing icons in the topbar on AIO

Hi, I’ve been writing a custom theme for my Nextcloud and I’ve hit a bit of a roadblock. I’d like to use custom icons for all the apps in the topbar:

However when i try to load them, either using Custom CSS or JS, all I get are Content Security Policy errors.

I’d like to know: is there a way to add custom icons (and other resources), preferably without adding/modifying anything in the core files? Maybe just simply by uploading to a globally shared folder? Or is that the only way forward, and if so would I have to worry about them being reset whenever I update the AIO?