Most of the textures for the GUI can be found within the folder:
\FeralHeart\media\textures
You'll see all the different kind of icons in there and you can modify them to your liking.
Most of the modding to the game is really a cut out and paste in new kind of matter with the exception of some cases needing some config changes. You can modify the music within the game, which markings you have within the game, which items that "you" can see. Some modifications are bigger, some are smaller and some require more or less work than others. Totally customizable is probably a lie, but you can do a quite a lot to the game.
If you go to the addons & mods section here on the forum you'll get a good idea of what can be done to the game:
http://www.feral-heart.com/index.php?option=com_jfusion&Itemid=2&jfile=index.php&board=34.0But as far as the GUI/Textures go, it's just to modify the pictures that can be found within the folder.