If you have 10k rep on AskUbuntu, you can see this answer, which I deleted, because I'm apparently wrong.Basically, a user's question was about how to change permissions of theme files in /usr/share/themes so they could be opened for writing.I answered that sudo chmod a+rw /usr/share/themes would do the trick, and admittedly gksudo whateveryouwannado might be the better option, but I probably take a lot of risks with my system that others don't with theirs (and got yelled at by muru for it).Specifics aside, due solely to the existence of themes in /usr/share/, I was convinced /usr/share was for stuff writable by all users, like themes, about which I stand corrected.So, where should packages / I put stuff that should be writable and readable by all users? (Pretending for a moment that security is not an issue, which it isn't for me.)
What's the convention for public files?
debian;permissions;shared folders
The point is, you should not create directories with write permission for all users. At least restrict the write access to a group of users. By convention, all local modifications of a system are in /usr/local/$dir. In your scenario I would advise /usr/local/share/themes.Do not be so quick dismissing security concerns. Take for instance, the example of a web server. Fully write-capable directories will be often be abused to upload scripts that are run with the privileges of the web server user (often www-data). If you are storing the themes with access to all users, they will be changed and subverted to distribute malware without any need to escalate to root.As I commented in the original question, I manage web servers with hundreds of virtual hosts, and what we do is create a group for each virtual host, then add users to that group.Will leave here a link about applying setuid and setgid to directories.https://www.gnu.org/software/coreutils/manual/html_node/Directory-Setuid-and-Setgid.html