Add Default as a valid option for GTK3 theme

This resolves Bug 2478

Signed-off-by: Slávek Banko <slavek.banko@axis.cz>
(cherry picked from commit f92343d24d)

kcm_gtk/kcmgtk.cpp

@@ -267,17 +267,17 @@ void KcmGtk::getInstalledThemes()
 		}
 	}
 	
-	bool gtk3installed = false;
+	bool gtk3installed = !gtk3Themes.empty();
 	widget->styleBox3->clear();
-	if(!gtk3Themes.empty()) {
+	if(gtk3installed) {
 		TQStringList otherGtk3Styles = gtk3Themes.keys();
 		if(otherGtk3Styles.find("tdegtk")!=otherGtk3Styles.end()) {
 			otherGtk3Styles.remove(otherGtk3Styles.find("tdegtk"));
 		}
-		gtk3installed = !otherGtk3Styles.empty();
-		if(gtk3installed) {
-			widget->styleBox3->insertStringList(otherGtk3Styles);
+		if(otherGtk3Styles.find("Default")==otherGtk3Styles.end()) {
+			otherGtk3Styles.prepend("Default");
 		}
+		widget->styleBox3->insertStringList(otherGtk3Styles);
 	}
 	widget->styleKde3->setEnabled(gtk3installed);
 	widget->styleBox3->setEnabled(gtk3installed);