I've actually been pretty impressed with LibreOffice as of late. It's fairly easy to adjust the theme (they have proper dark theme support now!) and layout to something pretty darn cozy feeling. Maybe for a power-user it's not enough, but for my simple needs, like fiction writing and simple documents, I honestly can't complain, they've done a solid job. Could it be better? Sure. But it's in a good place, IMO.
I think GIMP is a better example of a really user-hostile UX. That, almost more than any other open-source app, needs a UI overhaul.