My Inkscape has this issue where many icons are replaced with an ugly looking red cross (screenshot attached). I'm unsure why this is the case but I think it may be due to me using NixOS which doesn't follow the fhs. I tried installing it through flatpak and the icons look normal there but I don't want to use flatpak. Any help regarding how I could fix this would be greatly appreciated.

I'd check with the devs on the chat: https://chat.inkscape.org/channel/team_devel

Thanks Tyler! I'll try asking over there.

Installing gnome.adwaita-icon-theme fixed the issue for me!