gtk-2.0 and gtk-3.20 themes not entirely consistent?

I have quite a lot of widgets on the xfce panel, and although most of them are dark, as expected, they seem to have inconsistent background

For example, a few of the sensors have a black border around them, but not all.
WaveLAN does have a border, Network Monitor doesn't. Memory Monitor does, CPU Graph does not.

PulseAudio plugin and Notification Plugin have a darker background, as well as "Mount Devices".