Applies colorPrimary property to the window. The colorPrimary property currently changes the color of the Gtk.HeaderBar and it's children so that the application window can have a so-called "brand color".
Note that this currently only works with the default stylesheet that elementary OS uses.
the widget to apply the color, for most cases the widget will be actually the Gtk.Window itself
the color to apply
priorty of change, by default Gtk.STYLE_PROVIDER_PRIORITY_APPLICATION
the added Gtk.CssProvider, or null in case the parsing of stylesheet failed.