Your qt5ct screenshot indicates you have not set a kvantum theme. What theme do you use ? Does it support gtk-4.0 ? That is important.
You need a ~/.config/gtk-4.0 folder and symlink the “assets”, “gtk.css” and “gtk-dark.css” from your theme to it:
I use dark theme, so I set qt5ct to kvantum-dark:
In Kvantum Manager you need to choose a theme, in my case it is
KvGnomeDark.
Regarding the theme you use, you find a matching kvantum theme or not.