Modified circuit-connector-main.png
The circuit-connector "node" shadow came from Factorio' "base mod" gfx files named circuit-connector-main.png & circuit-connector-shadow.png that I "re-touched" adding the shadow to the original png.
I modified Factorio' circuit-connector-main.png and re-inserted it back into the Factorio "base mod" circuit-connector gfx folder to include the shadow. Maybe, it would be possible to add the "modified" png to the mods .zip and some code to "point" to the custom png. I would have done it this way but, as I didn't know where/how to add the "node" shadow to your mod code I didn't do it that way.
I don't know it's probably not the best way to go about it... I believe HD circuit network "shadows" are getting re-worked and updates coming sometime soon from Factorio' developers so this could change everything.
Well anyways, I'm enjoying your mod very much... Thank you for all the hard work you have done!