> And because every Linux distro uses a different theme and one of a dozen UI libraries/DEs/etc. the downloaded GUI application will look bad because it won’t fit in.
Most apps/DEs/etc. use either GTK or Qt. If your application uses GTK or Qt, it will use the user's preferred GTK/Qt theme by default, and fit in with all the other GTK/Qt apps on their system.
Sure, you might get unlucky, and your GTK app won't look great on a KDE desktop where that user doesn't have any other GTK apps and it sticks out. Or vice versa for your Qt app on a GTK-based desktop where the user hasn't picked a matching Qt theme.
But unless, for some reason known only to yourself, you decided to write your app for FLTK, EFL, wx, or some other niche toolkit, your app is going to work like at least one other app on 90% of users systems. And for those users running FLWM, Enlightenment, or some other weird graphical environment, I'm sure they'll cope.