Skills for authoring themes and blocks

That should be entirely possible and an agent using the skills and the example blocks from the shared theme should be fully capable of coding this.

I actually did a similar block a while ago. It’s not yet using the new Blocks API, but you could still look at the approach at Manuel Kostka / Discourse / Blocks / User Profile · GitLab. Looks like that for example, on the Canvas Central theme:

We don’t have blocks in core (yet). All blocks are just added using themes or theme components.

You can add blocks to existing BlockOutlets using themes and theme components and I think you need to be on a Pro plan or above to add custom themes. Else there should be no limitations.