Mp3tag has no skinning ability.
In fact, there's relatively little ability for the user to customize the interface, in comparison to many modern applications. Sure would like to at least have a customizable toolbar. With the recent work/attempt to support different resolution displays, you'd think that Mp3tag would at least have added the ability to select between larger and smaller icons in the toolbar.
There are just so many obvious places where the user interface could be improved, but Mp3tag's UI has changed very little over the years.