They are in the unity assets bundles.
Unity asset bundle extractor works, or at least the version I have does. It's a little tricky to use, but you can export - then change - and re-import textures into the resources asset bundle.
Here's the skin for Tango class.
http://imgur.com/a/RyeZg
What I haven't been able to do yet is get textures to work in the override folder, so you are limited to a modded bundle which is pretty inconvenient for distributing any skins, and when patches arrive etc... till someone figures it out.
I might try something else, I may have got the directories wrong or messed up the file format/name etc when I tried the override folder.