The themeability ran on anarchy but on the community-organized kind. Users expected apps to use the Desktop's toolkit and having their custom widgets respond we

The themeability ran on anarchy but on the community-organized kind. Users expected apps to use the Desktop's toolkit and having their custom widgets respond well to theme engines. But that's only for KDE nowadays, there was a period where GNOME was breaking theming with new releases and its maintainers argued for why they just supported the standard Adwaita theme and not anything else. KDE comes with a historic sloppiness at the individual application level, you'll quickly notice that padding, margins, lines, orientation are all over the place and it's just how KDE application developers roll.