Styles & KWin contextmenus: how to detect when a window/app uses a different palette?

[René J.V. Bertin]

PS: I don't really have an opinion about this: is it wrong for a window manager to ignore the fact that a window/application uses a different palette? I don't know any other WM (*) that adapts its decoration palette automatically to each decorated window's palette.
I could easily image a category of users who consider it wrong if the WM does not use the same palette for all windows, regardless of what's happening inside the windows. IOW, shouldn't this new behaviour be made optional.

*) In fact, I do ... Win10 has an option that does something similar, I don't think I'd call that an example to follow ;)

R.