Swing can look fantastic if you put in the effort as IntelliJ (based on Swing) shows. I'm not necessarily an IDE guy, but good luck with getting that kind of functionality with JS on Electron even remotely.
The problem with Swing theming is that you need to create a ton of classes "just" for theming your application. I did that years ago myself, but really Swing should have something like those libraries released years (>10) ago that had a bunch of premade themes as well as WYSIWYG editors for them.
There used to be many libraries (most of them were awful, half of them tried to mimic circa 2002 Mac OS X Aqua) so i do not remember any names, though i remember trying one with a visual editor that i gave to a designer at the place i worked at. We didn't ended up using it though because of the price.
