I have taken a look at your sample and it seems that the main reason for your problems is that you don't place the Skin attribute on the custom TextBox class, telling it to use the skin you are modifying. If the Skin attribute is missing, the control will default to the skin of it's parent class, the TextBox, and in there it can't find any matching template for your custom style, so nothing is rendered for the textbox.
You can fix this by modifying your class declaration to:
public class ITSTextboxGenerico : TextBox
this.CustomStyle = "ITSTextboxGenericoSkin";
Hope this helps,