Xfdesktop icons are not following the selected icon theme. This is seen easily atleast when adding a folder to desktop and changing icon themes Reproducible: Always Steps to Reproduce:
All icons are queried from thunar itself and looked up via the current icon theme. Watching the theme for changes and reloading the icons isn't currently implemented, and is a low priority.
*** Bug 1491 has been marked as a duplicate of this bug. ***
There is a related bug: I noticed that when for many icon theme, it does not show the theme icons except at the standard sizes: 16,24 & 48. I suppose that's because they don't provide scalable icons. Based on the principle of least surprise, you'd either have to scale the icons yourself or only stick to the standard sizes.
That might be GtkIconTheme's fault. Thunar-vfs forces me to use GtkIconTheme to lookup icons. Theoretically, the docs say that GtkIconTheme should scale icons if it needs to, but I remember back before I wrote XfceIconTheme (part of the reason I wrote it), it sometimes didn't scale if the size difference was over some threshold. I'm not sure what to do at this point.
ACK. I'd say leave it for now. It's hardly more than an annoyance.
Ok, it should watch for icon theme changes and update properly now.