the last version of plasma is here to prove your wrong.
open the color panel and tick the option "tint window title bar with accent color"... which used to be just the window decorator title and borders. ... now some dev wanting to post screenshots on Reddit decided the accent color should be a light pastel tone that can be applied to the top elements of the qt application, bringing back the worst of gnome 2 era.
I'm pretty sure I have that option turned on. I love the accent colour option (2 years ago in the anniversary release of plasma 5 I think) but it's only recent so not all apps have incorporated it yet.
But I don't really understand what goes wrong there.
open the color panel and tick the option "tint window title bar with accent color"... which used to be just the window decorator title and borders. ... now some dev wanting to post screenshots on Reddit decided the accent color should be a light pastel tone that can be applied to the top elements of the qt application, bringing back the worst of gnome 2 era.