Thank you. I do have Arcticons Dark installed from F-Droid (it's just called Arcticons there, it seems, while the other editions have their full names), and it's the latest F-Droid release of it from 3 weeks ago. I already assumed it would work like you described, but only the "Breezy Weather" and "Pixel" icon packs are showing up there.
I'm unsure what could cause this, but some possibilities I can think of are:
π€ Differences between F-Droid, Github, Accrescent and Google Play releases?
βGrapheneOS system security restrictions limiting the scopes of the apps?
If you (or anyone else reading this) happen to know anything that could lead to a solution, I'm all ears!
I'm not the one you're asking, but it seems to be this repo on Github and I read that it works because Jellyfin does/can(?) expose a Subsonic API.