Gtk-Message: 11:36:38.401: Failed to load module "colorreload-gtk-module"
Based on this answer https://unix.stackexchange.com/questions/612098/failed-to-load-module-colorreload-gtk-module/629998#629998 installing plasma5-kde-gtk-config helps, the warning is gone with it.
But I don't know where this dependency is missing. Maybe somebody would know and would be able to fix this minor annoyance.
Thanks, Yuri