This may be somewhat nitpicky, but I have noticed that several programs written with Qt do not respect the icon theme as set in xfce4-appearance-settings. All of my Qt apps still use the default Mint-X icon theme for toolbars, icons in menus, etc., which causes them to look a bit out of place compared to other programs. I have several icon themes installed; switching them in the appearance settings affects the rest of the system, but does nothing for Qt apps.
Example programs are: designer-qt4, transmission-qt
This doesn't apply to KDE apps (I believe these are written using Qt libraries as well), which seem to have their own icon theme altogether.
Looking at the transmission-qt program source code, the icons are returned by a call to QIcon::fromTheme().
I perused the Qt API documentation and opendesktop specs, but I couldn't find any configuration file that tells the Qt library where to get these icons from.
The qtconfig program looked promising, as it allows configuring many aspects of the appearance of Qt apps, but there is nothing there to set the icon theme.
I realize that I could probably rename the Mint-X icon directory and symlink it to the icon theme I want, but I think there's a better solution out there.
Oh, I'm running LMDE amd64 XFCE update pack 4.