Changing the bar colour of a Gtk.ProgressBar

GtkProgressbar.css

/*
 *  Operational System: elementary OS
 *  Programming Language: Vala
 *  Version Gtk: 3.0
 * ----
 *  var progress = new Gtk.ProgressBar ();
 *  progress.get_style_context ().add_class ("GtkProgress");
 */

@define-color progress_bg_color #37C8AB;

.GtkProgress.progressbar {
    background-image:
        linear-gradient(
            to bottom,
            shade (
               @progress_bg_color,
                1.3
            ),
            shade (
                @progress_bg_color,
                1.08
            )
        );
    border: 1px solid shade (@progress_bg_color, 0.9);
}