When I updated my system's KDE Plasma to 5.23.x, I began to notice that some applications don't follow the color scheme from KDE's settings. xosview now has the default blue background instead of the window background color it once had when I ran xosview in older versions of Plasma, Xaw-based apps like xfontsel use the white background instead of the scheme's window background color and Motif-based applications like nedit are now refusing to follow the color scheme set in the colors KCM. The one thing I liked about KDE in the past is that applications followed the currently set color scheme, and that included GTK-based applications like Chromium and Firefox, no matter the application style set in the KDE settings. (that's all the way back in the KDE 3 and 4 days I think, and it might be nice to see said feature return in a later Plasma 5 update or in Plasma 6)

Not sure what part of KDE was responsible for applying the color scheme to non-Qt and non-GTK applications, and if someone knows the exact component that made some X11 apps follow the current color scheme, let me know and I will file an issue in said component's repository.

And your distribution?