Glad you got the icons sorted out.
Fwiw, I would make that icon substantially different from all the others. Something like a big square button or switch or something, so that it's clear that it's a master switch for the ones below. Being so much like all the others is what I think makes it confusing. Either that, or put a strong border around each grouping of snap controls. Or maybe even both - border and master switch/button.
If you make a feature request, will you post the link here? I'll be glad to add comments.
Regarding changing the interface, you must mean this message:
http://www.inkscapeforum.com/viewtopic.php?t=10499#p50773. So if I understand, someone made that icon set and something called "gtkrc", which together can create that specific theme.
But there aren't any other themes available, right? It's a choice of one or the other, unless someone makes more. Right?
I was trying to look at the gtkrc file, just out of curiosity, to find out what it would take to make more themes. But it's a substantial page full of code. First I looked at the one attached to that message. But following the instructions given there, when I look at inkscape/etc/gtk-2.0, there is no gtkrc file there. Of course I'm looking at 0.92.2, and who knows which version was used when that dark theme was offered.
Eventually I found a pre-installed gtkrc file. (That message is a bit confusing -- I was trying to find all those directories, and it was only when I got to the end of the lists, that I realized you only need to find the one for your operating system.)
Anyway, the 2 gtkrc files are only vaguely similar. Even without knowing the code, I've found occassionally that I can make simple changes to codes, and get the result I want. But in this case, there are very few similarities. I think it would take an expert to make other themes.
I don't know if I ever got around to it, but I hope someday Inkscape can have a simple way to make and change themes or skins. I might have made a feature request..... Ah, indeed I did -
https://bugs.launchpad.net/inkscape/+bug/963576