Are there turnkey solutions or extensions that theme Lenny in whole or in part or are you rolling your own? I’m definitely interested in modifying the look of the site a bit. Mostly I find it overly narrow.
It's not possible at the moment. Lemmy devs acknowledged it was a widely requested feature but last I read, they were focused on maintaining the performance of Lemmy during the spike of users during the great Reddit migration.
The devs also explicitly mentioned this request in their statement:
we are seeing lots of requests to implement major new features, such as migration between instances, or combining similar communities. As described above, we are completely overloaded with work, and definitely won’t have time to implement these in the near future. If there is a feature you want to see implemented, you will likely need to work on it yourself, or find someone who can.
For now, you can create another account on another instance and use that instead (or both), or just peek how the experience might be.
Do I truly own my own data then? If I make a post, such as this, which is posted from lemmy.world to lemmy.ml, would my removal of the post on my instance remove it from lemmy.ml as well?
You truly own you data in the sense that it's not in somebody else's hands, but once you decide to share it with other instances, you should assume it's public for everyone. That's just how the internet works.