On the other hand, I think designers aren't that into contributing to FOSS - traditionally designers have been more on "paid" side of computation.
A pure designer cannot one day find themselves writing code for a major piece of opensource software.
A pure programmer can find themselves arranging some UI and designing a logo on Inkscape, that won't look and feel great.
Put the two together and something magical can happen, though not always, as design is so easy the programmer can be overcome with hubris and discard or ignore the designer advice and do the designing himself, with obvious results.
In a functioning software company, the managers recognize this skill difference and simply order the developers to implement what the designers suggest. They grumpily do. (It’s obviously not always so stereotypical. I’m exaggerating to make a point.)
In a typical OSS project, no one can give orders. The programmers ignore any good design suggestions because they don’t recognize those, and implement their own crappy ideas instead. Sometimes while arrogantly dunking on the designer who tried to make a contribution. Consequently, UX quality is abysmal, and anyone who could fix that is driven away. I don’t see a solution to this.
Sometimes making software more user-friendly may even cause the code to get more "ugly", because instead of a nice and elegant piece of software that only solves the core problem, you now have to deal with all the weirdnesses of how people perceive the problem as well. Taking away complexity on the user's side almost always adds complexity to the code. And if you're the developer, it's very easy to think that the tool you created is perfectly usable, because you understand everything about the mental model that went into it.
Generally, going into an open source project as a stranger and suggesting to make architectural changes without expectation of pushback is not a good idea. Any change there is going to trample on someone else's workflow. You have to build up trust first before people see you as an authority, that is the solution and it's often the only one. It's worked for some projects such as the recent post about Audacity, even though it's a bit of an extreme example because the person was already established as a well-known designer: https://news.ycombinator.com/item?id=26995610
I don't blame the programmer for dismissing suggestions they don't understand but it's also not cool if you were "dunked on" so I'm sorry if that was your experience. Keep in mind that's still better than some, a lot of projects will just ignore suggestions completely if they figure you're just not part of their target audience, and they don't have the budget to expand that audience.
Of course that's not a logical inevitability, things can change with more understanding. Your link actually makes me very happy, I hadn't seen this. Let's hope that this works out, and becomes a model that more OSS project can follow.
UI design in contrast is a top-down effort. You can't just mix and match various works from people. It's about information flow and consistency. A unified interface has to be agreed upon, and then implemented across the board. And any part of the system that fails to meet that standard, essentially breaks the UI.
User testing. Making user feedback the key driver of UI related decisions.
Brain computer interfaces… UI's as they exist today are merely an abstraction layer for getting the brain to engage with and process information.
Most people, let alone programmers, don't realize there's more to "design" than pictures.
Deviantart was a huge platform where people would share art and designs/skins. To some extent that still exists for iOS views.
I think one thing that's different about the deviantart model was that back then apps used to have skinning interfaces. And then designers could focus on just skinning. You could argue that gnome themes are like that. I would disagree, but also skinning isn't quite the same as doing ux.
UI/UX on Mac is a lot easier than UI/UX on gtk applications or QT applications.
On top of that, I don't think most designers understand OSS. It's basically a different universe and these two world don't often interact.
It's about the users and their ability to understand how to work with your software to address their needs and expectations... not about your code, the designer or the manager. Those people are the product, NOT the user.
If I had a dollar for every programmer and manager that thought they knew better and stubbornly cooked up a crappy MVP nobody wanted, I would be able to fund your OSS.
Right now I have ten crappy MVPs to donate and can put down $20 to sponsor others. I could probably have another 5 MVPs out within the next week if work is slow.