Being open source does not make it easy to theme it.
Yeah, I’m not suggesting it’s easy. They’re completely different web apps and Lemmy-UI is its own project that isn’t setup to be themed in such a drastic way.
Lemmy is built to interact well with a wide variety of web and native apps, so I don’t think we’ll see much effort putting into trying to make Lemmy-UI the answer to everything. Once the other options mature enough, instances could just switch their default web apps altogether.