Done, specificity was a bit low in my humble opinion.
Without the context of an UI modding site (wowinterface) as the starting point, it would not be clear at all what you mean by modifiable / customizable UI.
I mean 99.9% of games have customization options (from scale and movable elements to keybinds and video options).
If you meant player / community created customizations shared with other players as mods or addons it should have been more clearly stated and the focus of the questions could have been more directed as well.
Just my 2c.
