The icon files are in UI/Default/images/icons/. To use a custom one, you'd put it in UI/WhateverYourCustomUIIs/images/icons and keep the name the same (eg icon_is55.dds would stay named icon_is55.dds).
By making it blank, I meant making it transparent. But thinking again, Drums has the right idea of it, you need to edit the alpha layer and make it entire black, as that will make it transparent. Making the content (what you normally see) anything different would just overlay that on top, you have to use the alpha layer to make it transparent.
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