There is no easy way to find or install Themes or Plugins, except to manually download a file and install it to Mailspring via the menus.
It should be possible to browse, install, and enable/disable Mailspring themes and plugins directly from the UI, via a sort of “catalog” or “store”.
Right now, Mailspring has much of the technical functionality for enabling and disabling plugins. This would need to be polished up a bit and tested.
The catalog itself would be possible by having a version-tracked JSON files in a dedicated repository, that would allow anyone to submit PRs for adding new plugins and themes. The JSON files would provide the canonical (ideally GitHub or GitLab) URLs for the ZIP archives.
When Mailspring checks for updates, it could also poll these JSON files and check the installed versions against the available versions.