Those web apps are made with theming in mind? One thing that Old Reddit allows is subreddits having their own css theme, some of them are better than the default old reddit. Suppose I’m the owner of a Lemmy instance and want to replicate the look of a specific reddit community, would that be a pain in the ass to do? The same question for Photon and Voyager, as I imagine some instance might want to further theme it, by changing things more thant just the banner and icon.
These are all open source projects, so you can indeed fork the code to implement your own changes. You can visit the GitHub pages in the OP to see whether it looks like something you'd be comfortable with doing - but it's all definitely possible.
Wouldn’t it be better to write those interfaces as Lemmy-UI themes? Or is that impossible?
You could fork Lemmy-UI and build in those changes, but it would be pretty messy. These are separate projects being developed using different languages/frameworks/design philosophies. It's a bit like how there are many different Reddit apps on Android. You could theoretically just fork and reskin an existing project, but there's a lot to be gained from having something purpose-built.
Being open source does not make it easy to theme it. Reddit was closed source, but the tools to create a subreddit theme were far simpler than having to fork the actual visual code of reddit.
There are also open source projects that allows you to edit just the visual files in a simple manner, for instance, KDE's Plasma is relatively simple to create new themes without having to fork the entire Desktop Environment, because it was made with theming in mind.
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.
However, Lemmy-UI accepts CSS themes. If Old Reddit could have significant changes between subreddits using CSS, then it doesn't seems out of place that the same should happen with mlmym, or even Lemmy-UI.
It could, but I anecdotally think folks on the Lemmy side are more interested in using various apps and interfaces with completely different experiences beyond layout and theming. You may want to consider posting on the Lemmy-UI or mlmym GitHub project pages though.