Additional followup: a lot of trawling around the web suggests that when Thunar transitioned to gtk3, its styling became based on properties other than the standard ones set by themes. I found several web pages suggesting that one add Thunar-specific styling rules to the `gtk.css` file of one's chosen theme. My efforts to discover exactly what CSS classes or IDs to use were more or less fruitless.
Anyway, this looks like the sort of gnarly problem that Linux distributions have to solve: different upstreams (Thunar vs themes) making different assumptions about how things are supposed to work. Good luck! Norman Ramsey
