Gtk3 port theming

[flo.xfce at gmx-topmail.de]

Hi all,
in Landrys porting efforts of various panel plugins (and I have seen it
in other applications, too) I came across code like this:

css = g_strdup_printf("label { color: %s; }", gdk_rgba_to_string(color));
css_provider = gtk_css_provider_new ();
gtk_css_provider_load_from_data (css_provider, css, strlen(css), NULL);

This looks really hacky to me, so my question: Is this the recommended
way of setting widget styles independet of the user's theme? In Xfce
there are multiple situation where we allow overwriting of the theme,
for example the panel where you can set custom backgrounds. I'm just
curious what would be the right way of achieving this.

Kind regards