Feature Request: Theme data source

[Scrambles56]

It would be great if the provider was able to provide a theme data source. Looking at the fusionauth API docs, a theme should be able to be retrieved easily.

This would simplify working around the issue with creating a theme, where you want to have a source theme, but override some of the templates (see this issue). The work around could then be: use the data source to retrieve the source theme, and then provide all the original templates except the ones the user wants to change.