At present, the gtk-common-themes snap declaration has slots-per-splug: * set for its gtk-2-themes and gtk-3-themes slots. Please update the icon-themes and sound-themes declarations to also set slots-per-plug: *.
I had previously requested greedy plugging for the GTK theme slots here:
… and the same rationale applies to the other two slots: they are all designed to co-exist with other theme snaps.
As things stand, auto-connection of the two remaining slots can break if another snap providing the slot is installed (e.g. the icon-theme-yaru-mate snap). As we’ve now got a daemon set up to prompt users to install missing theme snaps, it’s quite possible for non-technical users to get their system into this state.