I think you can just inject the CSS yourself (assuming you use a reverse proxy for the interface)... of course that isn't an amazing solution for hollo- But from what I've read hollo doesn't plan to really have a big UI... "Hollo does not have its own web interface. Instead, it implements Mastodon-compatible APIs so that you can integrate it with the most of the existing Mastodon clients."
the more important thing is the software- and hollo is written in typescript? Which is interesting- I haven't ever seen anything written on this stack before- I guess this might be useful for some people- But I myself don't have any interest with bun and typescript at all...
I mean you could just configure the UI to litterally whatever with a well-written web server config.