I understand the adding mods to change the numbers for the items, but does this also change the look? I have heard some people have written things about how a robe looks a certain way, but it is moddable to another look. Please help me with what this means? What changes how things look? Thanks.

Nope, changing the mods in an item will not change the look of the item when worn on your character.

 

The only differences that some items might have is a different appearance when worn by a male or a female; but this is inherent in the model and not influenced by the modifications inside.

 

Hope this answers your question

Model only changes if you're using an item from opposing faction.

The whole idea of custom gear is not to change look, but to keep the same look and only upgrade the stats of your gear as needed as you level up