no package manager should put stuff into /usr/local, thats why its /usr/local. package manager should only put stuff into /usr/… not /usr/local. In the past some mainframes mounted these directory via nfs to do some kind of software management. It contains global stuff that is not managed by the package manager. install some software via “tarball and make” and it most likely puts everything into /usr/local/…
It’s the best location for your needs. /home/shared is a bad idea. /home is reserved for home directories not some shared stuff.
/usr/local is specifically what you are asking for. i would put themes to /usr/local/share/themes for example. chances are gtk will already look there. You can manage permissions inside /usr/local as you like. since the package manager doesn’t care for that directory there is no problem. giving write permissions only to root still is a good idea. it suggest to learn to use sudo when you are working on your system.