This doesn't fix the issue of Swing theming being hard. The fix would be to make it much easier, preferably with a visual editor.

You mean like Netbeans Matisse, or more like actually having a designer on the team?

I was talking about themes, not UI design. Think GTK themes (though not their implementation), not where buttons in a form will go.

"How to design a modern Java Swing UI inspiration using Netbeans"

https://www.youtube.com/watch?v=SpKANTHXwmo

As mentioned, having a designer on the team matters.

I wasn't talking about such a thing. I was talking about being able to create a "plaf" theme using a visual editor or at least some rules in CSS or similar script that applies to the entire Swing application (like Swing's own plafs do) and doesn't require creating subclasses. This has nothing to do with having a designer or not and it is orthogonal to it (you can give the tool/rules/whatever to a designer if you want).