KDE Plasma Desktop
Ugly widget problem
This problem is for me the main reason that I've stayed on GNOME, practically everything about KDE I prefer, but this ugly retro "Windows 3.1" widget problem that some applications seem to have is a real show-stopper for me. The strange thing is that the problem only applies to some applications. The desktop environment itself works fine, for example the window menus and task-bar menus are all fine, but some applications don't seem to be picking up on the theme preferences for some reason. You can see the difference in these two screenshots, the first shows the "Dolphin" file-viewer which is working nicely, the second is my Thunderbird email client which is using the wrong font, buttons and icons (click on the images to get the 1:1 view).