Instead of having every plugin function inside the same file, which
increases the loading time unnecessarily and is not fishy, source the
`uninstall.fish` file for the plugin before the `uninstall_$plugin_name`
event is emitted.
* On clean install, configure and download `default` theme.
* Do not allow `default` theme to be removed.
* When the current theme is removed, fallback to `default`.