I am doing a little development on Friendica recently but I'm certainly not a core dev or anything, and likely won't be in the future either, but I kinda feel like a barrier to this being the case right now, is that these themes are really extensively different from each other currently. It's not just different styling but potentially totally different content as well, that they support. So when making a change it's potentially quite a lot of work to support the other theme as well, so I kinda just focus on the one I'm using, which is just also based on which seemed the most maintained, and it's seemed to me to be Frio.

Ideally I actually think Friendica should focus on one and drop support for all others at such a deep level, or find a way to pull others out as an addon so they can be more independently maintained. Instead the theme could be changed to be a lot fewer changes, just in how things are styled, and not being able to rewrite everything so much.

In the limited changes I've made I HAVE made changes to Vier too when it's been relatively easy but just not every time. And I guess a secondary reason could be that since I don't use Vier myself currently, I don't have much of an understanding of how it "should" work or look, so it's perhaps a bit risky just to essentially apply the change from Frio to Vier without considering how Vier is fundamentally different.

Anyway I have no idea how much this is representative of the regular friendica devs but I can't really speak for them, so these are just my own thoughts about this currently.

Good question!

I forgot another reason: Since friendica is developed by volunteers perhaps people are also a bit more likely to just work on what they feel like makes sense, whereas things are quite a bit more rigorous when one is working at some company providing a service, and there are customers to satisfy, Q&A testers and management who have to approve and such.

Thanks for caring! I've only done some fairly targeted changes here and there, like just adding some CSS so images with "Alt" texts are marked with a tag indicating it, added a new and now functional way of updating one's profile picture by hovering over it, changed the contacts link from the eye icon to just make the text clickable and improved how addons and connected apps look when there is no content there and changed how the text is displayed in the browser tabs + added translations there, as right now it's always English no matter what. Most of the those have been merged so they'll be released the next time friendica is released, but not all of them.

One bigger thing I would also like to do, is remove a lot of the duplicate links to the same pages (e.g. contacts and calendar) spread out across friendicas many menus. I feel like it's quite confusing from a navigation standpoint right now.

But someone's who's really doing comprehensive changes to friendicas UI is the penguin @Alt Pygoscelis Papua, through the Bookface theme they created. (Technically it's a variant of Frio right now, but it changes quite a lot for the better.) My changes, or those that are merged anyway, can't really be tested unless one is on a friendica instance running the latest development version instead of the latest release, but bookface is released separately so changes are more frequent there + as far as I understand one can apply the theme oneself, without relying on the server admin to do so. So it's possible to test for non-developers, and feedback could probably help there!

Development of Bookface is happening here:
https://gitlab.com/randompenguin/bookface and on the "issues" page you can see some the upcoming planned changes, where one can also write suggestions