So there are two main things here - since this is a content interface we need an assurance from the snap publisher that they will maintain a stable “API” for the content interface and hence won’t remove things from it etc which could then break other snaps which use the content interface.
The second thing is since this is using greedy plugs to ensure it is auto-connected to any plugging snap then we need to get some assurance that the snap is “safe” (ie that it doesn’t ship say a daemon or hook or any command etc, and to vet the publisher as well).
FYI - these are all detailed at Process for aliases, auto-connections and tracks under “content for theme snaps”.
Looking at the current papirus snap it only appears to contain image files etc and the snap yaml only declares the icon-themes slot as named above, there are no hooks/daemons/apps etc.
So this all looks good from my side, +1 from me.
The one remaining thing to do before granting this is publisher vetting - @Igor could you please take a look?