I had found older threads dealing with this problem but most of them suggest changing KDE’s appearance settings which is really not necessary. So I created a new thread so that the info is not lost
Those using KDE may have had the problem that KDE overrides by default the GTK themes, thus leading to bright version of ardour3. In that case the console prints:
(ardour-3.0:4962): Gtk-WARNING **: Unable to locate theme engine in module_path: “clearlooks”,
Instead of having to change the system wide settings that GTK themes are replaced with GTK’s ones you can just install the clearlooks theme engine. And voilà, that’s it!