I just figured out why my textured background in the Ruby/Gtk2 application wasn't displaying... and, indeed, I couldn't apply any Gtk styles... on the old laptop with the new Kubuntu.
Kubuntu helpfully supplies a file called ~/.gtk_qt_engine_rc, which helpfully supplies default styles for all Gtk widgets.
The defaults were overriding any specific styles I tried to apply.
Removing that file makes my spiffy visuals appear.
Now I just need to figure out how to make my specific styles take precedence over the generic defaults....