Overlaying matrix objects (making the top one have a transparent bg)

Hi, I'm currently trying to overlay a set of faders on a set of buttons (the fader is used as a progress bar for a clip launcher). To avoid complicated debugging, I'd like to keep the matrix at a single level, so I figure the simplest way to achieve what I am after is just to overlay two matrices, and keep their geometry in parity using OSC. However, when I overlay matrix objects, the background of the top most object is completely opaque. What CSS selectors do I need to change to make the top widget transparent?

Oh, super simple:
:host{
background:none;}

on the top matrix object....