GTK Eventbox background Color visible on Synapse (GTK3)

[Kwaadpepper]

Synapse use what i believe are Eventbox extended objects.
The issue is the bg color is rendered transparent.

So obviously setting * { background-color: red; } affects its color. But i tried to affect all background-color props in gtk.css (gtk3 folder). None is affecting EventBox. I believe this may not be implemented by this theme ? Can help more since i don't know anything about gtk stuff.

My synapse version is compiled against gtk 3.16.7
I also tried the branch upstream-polish with no success