Listen on a Unix socket for "current theme" queries

[Samasaur1]

This will allow me to forward the socket over SSH connections, allowing remote machines to automatically update their dark mode when the local system theme changes

[Samasaur1]

swhitty/FlyingFox#143

[Samasaur1]

I could probably also use https://github.com/Kitura/BlueSocket

[Samasaur1]

I did manage to get this to work (code as yet unpushed) using FlyingSocks, but the issue becomes how often we query the socket in SSH connections. It could query every time a command needs to know the current theme (e.g. for aliases), but this would probably not work very well. It occurred to me that it might actually work better to use SSH local forwarding (the opposite direction from remote forwarding) and have some kind of "DMN Lite" running on every machine I SSH to, which listens on a socket, and then I local forward those sockets back to my Mac, and the central DMN iterates over those sockets whenever the theme changes, which notifies the remote machines