Well, you were close with the suggestion to look in the index theme text file.
The Issue is;
I'm trying to let Storm's icon show the symbolic files on both the main menu as the side menu of Nautilus. I know your icons (antares, azenis and prowler) and also the icons that I made to visualize those tiny icons do show them.
But the icons Storm made (neuron and nebula) do not.
I've been using his icons set as a base for mine, so my icons do show and the index theme files are the same. Although I copied the icons (nebula) in folder places and add the text symbolic to it they still don't show.
Eh, but I can test it on Z16 Lite no problem.
I am testing using Neuron Purple. On all other icon sets, I see Regular Icons in the Zorin App menu.
On Neuron purple, I see only Symbolic-looking Icons for the Places in the Zorin App menu.
Looking in the directory /Neuron-purple/places/16
I see only the Symbolics, none of the regular.
So, I opened Scalable in a new window, highlighted all icons, copied - pasted into 16 to make size 16 copies - then tested the icon theme - all icons in the Zorin App Menu display the regular Neuron-purple icons.
Does this imply that also the symbolic icons work included those in the sidebar on Nautilus?
If so, then perhaps its a fault on my side/Gnome. I guess Storm does not mind that those symbolic icons do not look like his nebula ones. I'll look no further.
Um not exactly... and I corrected my post earlier to try to be clearer.
The sized 16 icons in the 16 folder look like the symbolic icons, but are not the symbolic icons.
By placing the copies of the .svg files into the folder, then they replace those, making the standard look consistent, but without replacing the existing Symbolic icons.