I noticed a Theme.data field in the Javadoc, and testing it via REST it appears to be a custom data field like the one User has, but it is not documented on the main API page: https://fusionauth.io/docs/v1/tech/apis/themes/

So, I just want to check if this API can be relied on and this is just a case of missing documentation, or should I avoid this field?

The use case is checking if we can write one 'master' theme to share among applications/tenants, and control specific customisations via feature flags stored in the theme.