People like OP and me would like that, but from the POV of the lemmy devs, that’s quite a shit PR, isn’t it?
Well you gotta do some integration work obviously instead of just changing the theme outright. Maybe add in a easy way to set themes and layout. Default will be whatever main Lemmy devs think is good, and optional settings that allow a style that OP wants.
It's simpler than creating a whole separate client and then also having to maintain it whenever Lemmy's backend changes or when they introduce a new feature.