sprite-button with semi-transparent sprite

[PFQNiet]

I'm creating some custom GUI for a part of my mod, and I would like to be able to replicate the visual effect of a filtered inventory slot.

To this end I created a `sprite-button` with `style="inventory_slot"`, but obviously just setting the `sprite` to the desired item results in the sprite being shown -that's what it's supposed to do after all.

Is there a GUI style I can use to make the sprite appear slightly transparent, as it does on filtered inventory slots? Or some other trick to it that I'm unaware of?