If i steal 1 leather, what happens to the other 100 leather i also carry. Also, do you steal ingredients? Thanks!

Only items that are actually stolen will be considered stolen. The taint of one stolen items does not get on others in your inventory. The only way the flag can get assigned to an item is to put it in a container that is owned by someone else then taking it out later. Ownership can, but does not always, get transferred to the container's owner that way.

Yes, you can steal just about anything in the game that is owned by someone else, including ingredients.

Well thats definitely a step forward from Morrowind.