It may be more trouble than it's worth to match icons on KDE. Anyone who really cares can set the GTK theme to match KDE, so perhaps you could just use the Gtk icons?
upp has picked gtk for x11 widget rendering, so using its icon theme api is the logical thing to do.
There is just no way to do the right thing for upp in case of KDE/GNOME theme mismatch. And KDE will screw the ISV with their bi-annual incompatible rewrites-from-scratch anyway.