[FEATURE] Make it easier to add user-defined themes

Is your feature request related to a problem? Please describe.

Make it possible to add a user-created theme without having to change the code (like https://github.com/unixorn/lima-xbar-plugin/pull/34 did).

Describe the solution you'd like

Ideally something where files in ~/.lima-xbar/themes or ~/Library/Application\ Support/net.unixorn.lima-xbar/themes can be selected from.

It would be nice if there was a Load Theme command that you could give an URL and a Theme Name and the plugin downloaded the theme and installed it as a file so that users don't have to rummage through ~/Library

Describe alternatives you've considered

Using defaults write to define the various states. This seems way more annoying than dropping a file into a directory or downloading one