"jsx isn't enabled" error when using a Gatsby theme

[dungle-scrubs]

Hi,

I'm developing at Gatsby theme and everything works great within Yarn Workspaces. However as soon as I plug the theme into a Gatsby site, I get the error:

Support for the experimental syntax 'jsx' isn't currently enabled

I've read that out of the box, Gatsby doesn't need anything special and should just handle everything, so there's no immediate need for gatsby-preset-babel, unless you need to change something.

I've also read that gatsby-preset-babel excludes node_modules by default. In this case it would make sense because the error is coming from jsx within the Gatsby theme itself, not the Gatsby site.

Assuming I'm correct about the issue, how can I get Babel to transpile jsx inside a Gatsby theme within node_modules?

Here's my test site's repo