Check all of the gotchas that've been mentioned above. Make sure you're using ZIP compression, make sure you ZIP from within the directory where extension.xml is, make sure you don't have hide file extensions enabled, etc...
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OK, I have been using the default zip from Mac/PC, let me see if downloading 7zip works. I've confirmed that there are no other extensions.
Does this work in FGU?
I have a interesting issue here.
I had this working in my FGC and I just copied the extension to Unity. This should be enough, right? Well, it's not working. Since I feel I know how to do this, both from experience and by reading everything above, I tried to delete the extensions and start from scratch. Didn't work also.
Funny thing, I was just cycling through the different themes to see them all, and I activated the simple dark theme from FGC. A bunch of errors appeared in my log, but FGU started normally with the simple dark theme active. The Icon change works with the FGC theme in Unity, but when I go back and select a FGU theme like the Light/Dark Theme the custom Icon disappear again.
Any idea what I may be doing wrong?
The theme is probably loading after this extension and replacing the replaced icon again.
You are correct. Just tried without any theme and it worked. Curious that FGC themes doesn't provoke that, and FGU themes replace it.
Any way I can counter it?
Just wished I could select load order like game mods! :-)
Anyway, let me know if I can do something about it at some point.
Thank you damned.