I'm not sure customizing the theme will cut it. Because it's usually a background process queue that handles adding messages to an outbox and making them available for subscription by other users and instances.

So I think support needs to be integrated into the software.

Right, I partially misunderstood your idea. For notifications and such you would need to modify the source code at a deeper level. But for just customizing the home feed (update steam?) changing the theme might be enough.

Ideally there should be a personal ActivityPub instance implementation, something that doesn't require hosting all Mastodon content and can be easily customized in various ways.