For simple UI you need to understand how layout works and then use the drag and drop designer. IMO is much simpler then learning and debugging CSS issues.

If you want to create custom elements I assume is simpler to do it in html with a couple of nested divs, background images and some css rules and some trickery and most probably your custom widget is not accessible and missing some nice features. As an example I know toolkits(Qt,Flex4) that can render huge tables of data, allow customization and are very efficient and you don't need third party libraries for that.

Is much easier to understand CSS than QT designer, that gigantic generator of errors and wrong previews.

...and I have 5 years of professional experience as a JavaScript developer, yet I strongly prefer to write desktop apps and utilities with Qt (both PyQt and C++/Qt) than Electron.

You do you, I tried to make a simple list on PyQt and everything went wrong, a lot of signal issues, a lot of debugging compile issues, missing dependencies, wronly versioned dependencies despite downloading QT straigt from the site, never again going away from CSS and JS. And with flexbox and SASS is a pleasure to write CSS anyways.