It has nothing to do with patched GTK. It has to do with a bug in GTK that is present in all versions of GTK+ 2.x. Ardour from ardour.org uses its own theme engine, not any provided by your system.
Why don’t you show us the precise error message, and comment on whether it totally stops Ardour from running?
ardour: [INFO]: Loading color file /usr/share/ardour5/themes/dark-ardour.colors
Gtk-Message: (for origin information, set GTK_DEBUG): failed to retrieve property `gtk-primary-button-warps-slider' of type `gboolean' from rc file value "((GString*) 0x345aaa0)" of type `gboolean'
Gtk-Message: (for origin information, set GTK_DEBUG): failed to retrieve property `gtk-primary-button-warps-slider' of type `gboolean' from rc file value "((GString*) 0x3465700)" of type `gboolean'
Gtk-Message: (for origin information, set GTK_DEBUG): failed to retrieve property `gtk-primary-button-warps-slider
(ardour-5.10.0:21223): Gtk-WARNING *: Unable to locate theme engine in module_path: “adwaita”,
Gtk-Message: (for origin information, set GTK_DEBUG): failed to retrieve property gtk-primary-button-warps-slider' of typegboolean’ from rc file value "((GString) 0x2499420)" of type `gboolean’
(ardour-5.10.0:21223): Gtk-WARNING **: Unable to locate theme engine in module_path: "adwaita
(ardour-5.10.0:21223): Gtk-WARNING **: Unable to locate theme engine in module_path: “adwaita”,
(ardour-5.10.0:21223): Gtk-WARNING **: Unable to locate theme engine in module_path: “murrine”,
Both errors will repeat until I press Ctrl-c. Ardour never opens.
If I change system theme to ‘Breezy’ both demo and distro packaged version work fine.